If the Android is outdated on a phone and refuses to install the newest apps, then LineageOS https://lineageos.org/ might be a solution.

Other reasons are: Getting rid of all annoying pre-installed applications and features, creating a simple phone as for elderly people, ...

Android consists mainly of the following parts: The open source part, device specific drivers and the google part containing playstore, gmail and others. linage-<version-device>.zip is the image to be "sideloaded" into the file.

Due to issues the google part is not part of LineageOS but can be installed as well.

LineageOS has its wiki https://wiki.lineageos.org/

The supported devices are https://wiki.lineageos.org/devices/

Mostly newer devices are supported, older as the Samsung Galaxy S III (International) (i9300) get no more maintained. Luckily under the wiki device list also those devices are listed where a guide how to install them can still be found.

LineageOS has for the supported architectures (as arm or arm64) two zip files under Download Extras https://download.lineageos.org/extras. One is to get root and one to remove root. So no strange untrusted apps need to be used. In the recovery mode and for example using twrp those zip files can be installed.

LinageOS comes with Trust that shows if root access is disabled. If Trust claims that SELinux is not enabled. It can be enabled starting in Rocovery mode then under Advanced there is a terminal in this rooted terminal. sestatus or getenforce shows the SELinux status, setenforce Enforcing