There are custom operating systems for phones but all of them are supported for specific models and may stop being updated after a while on older phones because development depends on volunteer work. Also you need to know how to unlock bootloader for them and may brick the device if you do something wrong while flashing the rom, so be sure to use a guide the first time. The main two custom roms I know are GrapheneOS(only for Pixel models) and LineageOS.
Edit: This is useless for OP though since bank apps don't work if they detect you are using a custom ROM.
It is good stats for the price but I am ticked about two things:
-Battery life starts out good but goes to shit fast
-MIUI sucks and it is a specific pain in the ass to unlock bootloader. My other phone was much older but I only needed to pass the unlock command from ADB, this wants me to use an app that only works on Windows and then wait a week.
YMMV because my phone is the cheapest model possible.