Interlining for curtains

I'd never heard of an 'interlining' - can you use separate linings that you buy (dunelm and other shops) and add to pre-lined curtains using curtain hooks?
I did this a few years ago as I'd bought pre-lined curtains (lining sewn in /attached), but they let the light in during bright summer mornings.
I bought separate black out curtain linings and attached them to my new lined curtains using curtain hooks.
This did a great job of blocking out the light and keeps rooms cosy in the winter.

I send my curtains to the dry cleaners, but have safely & successfully machine washed the linings and dryed them on the line outside.

I have both lined and interlined curtains. Interlined are warmer, certainly, but the main drawback is that cleaning is a problem. A lot of dry cleaners won't deal with them. This may or may not be important to you, but it's a consideration.

It makes the curtains more luxurious and warmer, but as you say there is an additional cost. I would have interlining for preference.