A lot of people don’t update their calendar as it’s quite cumbersome. As @pietkuip says, your confirmed sits are automatically struck out but you’re likely to have other times when you can’t sit.
If you put ‘calendar’ in the search field you’ll find lots of posts to assist you. Here’s one of them:
As for favouriting sits you’d like to be considered for in the future, you should be alerted when new dates become available (I think) but you may find some owners will contact you to invite you to sit for dates you may not be able to do.