My bro gave me this phone Samsung J7OOF/ds he told it developed some fault that I should format it and he ordered a new battery on Jumia.
Originally the phone has a nice specs 16gb ROM, 1.5GB RAM, 3000mAh, 4G LTE etc.
When I switched it on, I found of settings>accessories is not opening.
Then I checked the storage it shows 10.51gb free, but the File Manager showed a total space of 1.5gb (practically true).
then I downloaded some apps to check the phone info, they all showed 1.5gb ROM, 466mb RAM(but under settings>apps>running apps it's still reading the original RAM of 1.5gb).
It occasionally blacks-out when I try to on it back, it will show the boot screen with the screen I was working on before the blackout in the background.
The battery drains quickly sometimes it jumps.
PLS what is wrong with the phone and what should I do?

I was thinking maybe I should install custom firmware

samegold:
I was thinking maybe I should install custom firmware

Don't even bother yourself. That Samsung galaxy is a clone. In order words it's a fake Samsung galaxy.

You can do a factory reset and restore your phone's initial settings. In this way, you will get rid of all the third-party apps that bloat your Android.

Factory reset steps here https://www.unlockunit.com/blog/bypass-google-account-verification-factory-reset-protection/

Your brother noticed the same issues, probably that's why he asked you to flash it

Fake****

There are times when we Factory Reset our phone and forget the Google Password. Google Password or Gmail ID Password is the most important thing.
https://frpbypasss.com/