Chipping in with another short answer:
To WiredTiger, indexes and collections are not very different and they are treated the same way once they’re loaded into RAM (at least in the current MongoDB version). They’re just stored physically in different WiredTiger “table type” to optimize for snappy compression (for collection) or prefix compression (for indexes). Once loaded into the WiredTiger cache, both of them have a different representation vs. on disk.
For WiredTiger to work on anything, they would have to be loaded into RAM (WT cache) first. The OS will take care of caching them in the filesystem cache.
Hopefully I’m not introducing any confusion with regard to previous replies