> I could not tell you now how to implement a custom IQueryable

So what? I see LINQ used all the time, and it is almost entirely (extension) methods IEnumerable<T>

Could I implement IEnumerable<T>? I think I did once, as an exercise. It's not that complex. Not that interesting to be able to do it either.

LINQ is useful without EF. LINQ is not EF and EF is not LINQ.