Android is one of the main operating systems (OS) you will find on a smartphone today. It was developed by Google to compete directly with iOS, the operating system found on iPhones. One of the main reasons why Android was so successful was because it could run on many devices thanks to the Open Handset Alliance (OHA), an initiative that would allow manufacturers such as Samsung and LG to develop smartphones that may look and feel different, but still at the same time run the same operating system (with modifications made by the manufacturer).
Google may have prevented a monopoly that may have otherwise existed should Android have never existed, however it isn't all without some catch. Google is notorious for collecting large amounts of private user data, whether you're on a desktop, laptop, or smartphone. On all (Google certified) smartphones, there must be a Play Store and some Google apps (varies between devices). These apps, while useful for a large majority of users, can extract extensive amounts of information based on the user's search history and etc. However, there are solutions to prevent this and hence re-iterates the need of a guide such as this one.
If you want to buy a phone or you have already an Android phone you can check the links below if your phone will be supported.
Check the links below for your phone brand like Samsung Galaxy Note 4 and your model name like N910F.
On some devices, it is necessary to open or unlock the bootloader before you can install a custom recovery, root or custom ROM. Unlocking the bootloader usually requires the phone to be wiped of all data, meaning that you'll have the backup everything. Not all smartphone manufacturers allow the unlocking of the bootloader, so if this concerns you, you must do your research before purchasing your next smartphone. Although uncommon, some manufacturers package an “eFuse - Wikipedia” command in their products, which could brick (permanently damage) your device.
The best way to find out is to have a look at XDA-developers.
A recovery is an independent, lightweight runtime environment that is included on a separate partition from the main Android operating system on all Android devices. As the name suggests, the main purpose of a recovery is to help recover your device to a working state, however it is possible to make your device soft-bricked (unable to boot) if the wrong actions are performed. You can boot directly into recovery mode and use it to factory reset your device, delete the cache partition, or apply software updates.
Over the years, TWRP is the most used custom recovery. A custom recovery is necessary to flash a custom ROM and hence TWRP is a suitable solution if your device supports it.
TWRP supports touch interactions, encrypted storage, MTP (mass storage), and many more features to satistfy the needs of a power user.
Search for your device here to get TWRP.
More info about TWRP - here
Clockworkmod (CWM) was the de-facto recovery environment used by savvy Android users until TWRP gained traction in the smartphone market. Its interface was very minimal, coloured text on a black background. Controls were handled using the volume buttons and the power key. It is no longer the recommended recovery to use unless you have an older device that doesn't support TWRP (e.g. Samsung Galaxy S).
How to install on your device - here
We are all familiar with the time where your phone decides to play up and refuses to do what you want it to do. Anecdotal evidence has found that all smartphones degrade over time due to heat, stress, wear and etc. However this doesn't mean you can't still make improvements on your phone.
One common cause of slowdowns on your smartphone is the presence of bloatware or "bloat". These apps are unneeded and unwanted, meaning that they take up your important storage space and memory. A custom ROM however never contains any bloatware. This will mean that you will be able to reclaim any of the lost storage space and memory and use it for more important applications. This will translate to better performance (and sometimes battery life).
LineageOS isn't the only custom ROM available. There are heaps of other ROMs you can choose from (Resurrection Remix, CR Droid, Paranoid Android, etc.), each coming with their own unique combination of features and apps. To get started, it is best to consult on a thread over at the XDA-developers forums (Wikipedia). XDA is the first address of all your Android needs.
Like other desktop-operating systems (Arch, Debian, Mint, Ubuntu etc.) you have to find out for yourself which ROM suits you best.
LineageOS is an open-source operating system (OS) for your Android device. In the development community, it is often referred to as a custom ROM (or sometimes aftermarket firmware). Android is generally open-source (AOSP - Android Open Source Project), however the Google Apps (GApps) or other manufacturer (Samsung, HTC, ...) applications aren't.
LineageOS has their own option to get root access on your phone. Just flash the zip through TWRP. (later more)
Resurrection Remix OS is based on LineageOS with much more settings / features.
Root is required if you want to have full control of your device. It's necessary for certain apps like AFWall+, XPrivacyLUA etc. (more on this later)
AFWall+, for example, is using the iptables firewall (Wikipedia) which you can only be modified if your device is rooted. It's like the administrator of a software.
You have to be very careful to use the right root applications. Check the mantainer, date (how long is it available) and if it is open source! A root app also has full control over your device! You must remember this whenever you install a root app.
Rooting your phone typically voids your warranty, however this can be reversible depending on a case-by-case basis. We have never had a problem, but that does not change the fact that some manufacturers may not outright accept your phone for repair should it ever be necessary!
Over-the-air (OTA) updates are not possible with root. This means that your device will no longer receive updates from your smartphone manufacturer. This brings us to the use of custom ROMs. LineageOS can support your device (security updates, etc.) even if your device is not longer supported by your smartphone manufacturer! All new security patches straight from Google (Android Security Bulletin) are included in all official builds of LineageOS.
Applications such as bank apps may stop working when they detect a rooted phone. You can still use your web browser or a laptop/desktop PC, however there are solutions such as Magisk hide that can hide root for specific apps.
But root is not necessary!
You could go with a custom ROM but there is no need to use root. For privacy reasons, it is easier to use root, but there are some options without root.
A project overview. Perfectly usable for all your privacy needs.
F-Droid is the open source software store for your Android phone! It's available since 2010. Actually mostly every alternative is available.
Aurora store isn't really an application store. Aurora is getting all the apps directly from Google Play Store. Aurora Store is available at F-Droid.
Check microG. It's a good combination.
Magisk is now the most used way to get root access.
Back in the days, it was Chainfire's - SuperSU, but it was sold to a Chinese company and was no longer trustworthy. Magisk is open source and became more and more popular after the years. Magisk is actually more than just to get root access. It supports modules which could provide apps, tweaks or services. Some applications (bank apps/NFC/games/etc.) will not work if you have a rooted phone, but with Magisk it possible to hide the root check of the certain apps. This hiding capability may eventually stop working as Google makes these checks stricter.
Official Magisk forum thread - here