Lookup & add multiple records from a child table and add them to a parent table record

np, but the set up you actually need is what's called a many to Many Many join table called "Film Categories".

The setup is that

One Film has many Film Categories.

One Category has many Film Categories.

When you set up that joint table, you may find it a bit strange that you don't actually need to initially create any fields on that table. When you create the relationships, you will bring down the field name for the film title as a look up down to the joint table and you also bring down the category name as a look up down to the joint table.  Then on the films table, you would include in a better report of all the categories. And conversely, you can do the opposite. You can have a list of all the film films on the categories table.

Then you can also use a combined tech summary field on the category name to roll up all the Categories up to the parent film table if you want to be able to use them on reports and there's not a crazy high number of Film Categories.

Once you have that set up in place if you find it way too slow to add these film categories one by one there are some techniques to make that go faster, but one step at a time .....