My first question is about different drives. Could I purchase two different brand drives and use them with btrfs? (I assume yes)
You can.
2nd question: how does the replacement process go? Like if drive A died, so I remove it, and put a brand new replacement in. What do I have to do with btrfs to get the raid 1 back going? Any links or guides would be amazing.
Depends on what NAS/Software you have. If your NAS supports hot-swaps you can just pull out the defective drive and plug in another. Otherwise you’ll have to shut it down, swap the drive and turn it back on.
If you have already have the spare drive ready and you have slots availible, you can run a “hot spare”. This way you can even start the raid rebuild if you’re not physically near your NAS (like when a drive fails while you’re on holiday or sm).