what is the main difference between IEnumerable and IQueryable, which one should be preferred?

The main difference between “IEnumerable” and “IQueryable” is about where the filter logic is executed. One executes on the client side (in memory) and the other executes on the database.

For example:

we can consider an example where we have 10,000 records for a user in our database and let's say only 900 out which are active users, so in this case if we use “IEnumerable” then first it loads all 10,000 records in memory and then applies the IsActive filter on it which eventually returns the 900 active users.

While on the other hand on the same case if we use “IQueryable” it will directly apply the IsActive filter on the database which directly from there will return the 900 active users.

For more differences Link might help you.