What is a two-level cache?

Question

caching

It is necessary to implement a two-level cache, the first level stores data in RAM, the second - on the hard disk. I don’t quite understand the interaction of these levels with each other, but there is an assumption: writing occurs only to the RAM, but if the memory runs out (in this case, we set the limit ourselves), then we write to the hard disk. And what happens to records whose lifetime has expired, are they deleted or go to the hard drive?