Advice on MacBook pro retina 15 (2014)

Hello everyone,

My cousin has gifted me his old MacBook.

Appearance wise, it is in pristine condition. Not a single scratch.

Only issue is the battery. It is completely dead.It won't charge anymore. I've found a place to purchase and replace it. So I I'll get it done.

Since I got it, I have updated to the latest MacOS available (BigSur). Had some struggles but managed. I have not made a clean install. Just upgraded.

So here comes where I humbly request your help: I mm a windows/linux person. So I know my way around formatting and maintaining those systems. But iPad 2 aside I've never owned an apple product. I honestly am very lost on the know how for anything. From what I've seen here, the device is compatible with the newest Os. Is that so? Anyone knows a how to update your OS for dummies guide they can point me to?

Also, since I'll get it open to replace the battery. I've researched that it runs a nmve SSD. Can I replace it with a newer model? Or will I have compatibility issues.

Any advice is welcome. Thank you so much for your time and patience if you've made it this far. I'm sorry any mistakes, English is not my first language.

TLDR: Anyone knows a good guide for an apple dummy on how to upgrade my MacBook to newest MacOS?