When you share content with a Campaign, do players get to choose races that are in the books you own?

They can use all options in any book purchased by anyone in the campaign.

But this is only for characters they have in the campaign.

With content sharing enabled, players in your campaign have access to every character option available in the book you own.  That's technical access.

However, in terms of running the game, if you're the DM, you're free to impose limitations on what options players may use for characters in the game you run, or have a discussion where parameters are set.  If there's something you don't care for or don't want to double check like say Custom Lineage in Tasha's or what have you, you have the prerogative to say no.  Some players may push back with some sort of player agency derived claim that they should have options to everything presented in the character maker under your campaign.  Again, if you're DM and you own content you don't want the players using, make that clear up front or have a conversation coming to a consensus.  I'd say in such a negotiation a DM claiming they're just not comfortable running or designing a game around particular character options should pretty much trump the hardline player agency argument.

I was asking, cause when I got extra character classes, someone who was in the campaign could not see those options. My guess is they have to create the character from the campaign page, but I'm not sure. 

Yes, content sharing is only shared "within" the campaign.  The player has to build a character within your campaign to use the content you're sharing.

Once they join the campaign they can create a character with any available content that other campaign members own as long as one person has a master tier account. They can also join the campaign with an existing character after which the character should have access to the content.

What about if your a player and someone uses for example your race and you leave the campaign. Can they still use your race with you gone?

Unless it's been changed for 2024, yes they could keep using the race since it doesn't change. For things like subclasses though, if memory serves, they can't level them up without owning the subclass.

Quote from BigHeartedGuy >>

Unless it's been changed for 2024, yes they could keep using the race since it doesn't change. For things like subclasses though, if memory serves, they can't level them up without owning the subclass.

 
Depends. If they have selected the subclass, they can continue levelling that subclass even after leaving campaign. If they have not yet selected subclass after leaving campaign, then they cannot.