if you’re born into a religious family initially you just adopt it

Right. Because there's no inherent reason not to do so. And little kids tend to want to follow along with what they're elders are doing.

Compare it to how most kids initially believe Santa Claus exists because they were told so.

Kids are told that they get presents by pleasing their parents. And then the decision making / agency is displaced onto a fictitious figure. That's a very neat analogy for religion in the aggregate. Whether or not you "believe in Santa", you're still getting gifts based on your parents' resources and generosity. If you want the newest kids' favorite widget, you're following the letter of the law whether or not you adhere to the spirit.

You can also participate in some church community stuff without being a member or even going to church.

If you've got friends/family who are members/do go, sure. Because they're your social connection.

But you'll struggle to join a community event if you don't know anybody - or even when/where the event takes place. Nevermind knowing what's in the works, what needs volunteers, what needs money, and who is in charge of leading them. The more you want to participate, the more you need to attend the religious church functions. The more you want to get into leadership, the more you need to demonstrate your piety.