Yes there's a schema, but it's hard to maintain. You end up with 200 separate code locations rechecking that the data is in the expected shape. I've had to fix too many such messes at work after a project grinded to a halt. Ironically some people will do schemaless but use a statically typed lang for regular backend code, which doesn't buy you much. I'd totally do dynamic there. But DB schema is so little effort for the strong foundation it sets for your code.
Sometimes it comes from a misconception that your schema should never have to change as features are added, and so you need to cover all cases with 1-2 omni tables. Often named "node" and "edge."
We just sit a data persistence service infront of mongo and so we can enforce some controls for everything there if we need them, but quite often we don’t.
It’s probably better to check what you’re working on than blindly assuming this thing you’ve gotten from somewhere is the right shape anyway.
The adage I always tell people is that in any successful system, the data will far outlive the code. People throw away front ends and middle layers all the time. This becomes so much harder to do if the schema is defined across a sprawling middle layer like you describe.