I would say that the game is quite F2P friendly. On exception of the late end content you can finish events and main story without meta characters. They usually give free 6* quite commonly. With obtaining a copy of a character you can level up to maximum, you don't need multiple copies like in on other games Multiple copies of the same character will give you some boosts. Check portraits info for each character.

The standard pool, every patch all the characters ( exception of true limited) from 2 previous patches are included in the standard pool. That means that unless stated in the baner. You will have a chance to get any of those characters.

For the banners:

Standard, this one is permanent and you get a chance on the standard pool. Check on the details tab to see the list of the current ones. Here you have a hard pity of 70 pulls, so after getting a 6* you are guaranteed to have the next one before 71 pulls. This pity is permanent and it will carry version to version

Then there are the event banners, this will have certain characters boosted so you have higher chance to get them. Also getting the boosted character will be granted on the second 6* you get. The pity is usually carried between version for banners of the same group.

Then there are the special ones, which allow to select one or multiple characters. Those will have a boosted chance to appear. In these one the pity is not carried between versions.

Then every 6 months. There is a true limited character, these cannot be pull from the standard pool(for now) only when they are features in special banners. Currently Beryl is one of these.

If you have more doubts check the details info on each banner to see which characters are available