If I purchase the premium and choose monthly, would it automatically renew as soon as it expires or do I have to manually renew it?

It will automatically renew, but you can cancel it as soon as you buy it so it doesn't renew. Canceling your sub does not take away your benefits before the sub is supposed to end.

​

For example, in csgo, the players you put in matchmaking block, do they get unblocked?

I'm not 100% sure how it works, but I don't think that it keeps blocking them.

​

I also realized you can change your username with the premium so does that become available immediately after subscribing?

Yes. This is however only available with the full premium, not just game subscription. You can change your name with premium every three months.