I see that you are not familiar with mining, but don't worry, do a lot of research, youtube have lot of videos about
Look at this channel:
https://www.youtube.com/channel/UCVVWXoQfMfQVuzxcLylq9aAIt's very good to understand more about
In theory you can mining BTC with GPUs, but the difficulty is so high, that you won't have any profit, nothing, because there are Asics (machines that only mine BTC, and they are very efficient), so forget about mining BTC
You can mine any coin and convert to BTC easily
You are mining with Nicehash, right? Nicehash pays you in BTC but you are selling your hashrate. You are not mining BTC directly, understand?
About tweaking the cards, it's not only using MSI afterburner. If you want to mine with more efficiency, and gain more money, you can change the bios of every card, one by one, reaching the sweet spot of officiency (Hashrate per watt)
There are some tools that allow you to change this on the fly, without have to change the bios, but I've never used this, you can research about
And don't try to understand all now, before having your rig, wait for your rig and them start to learn with practice, it's hard to understand only reading about