Can someone direct to a thread that explains how to enter a plain photo exchange?
Depends on what you want and what's currently active. At this point, I think the follow activities are fairly active/current:
* Postcard exchange; instructions of the most recent round are here: https://www.photrio.com/forum/threads/postcard-exchange-round-60-sign-up.201488/unread But please note that you'll have to wait for round 61, which isn't open yet. Keep an eye on the Postcard Exchange subforum here and sign up if #61 is announced: https://www.photrio.com/forum/forums/postcard-exchange.41/ The Postcard Exchange is one that very explicitly is open to digital prints as well.
* Alt. Print Axchange. While this can be done without a darkroom, the prints do need to be non-digital ones, so that one's out. Sub-forum here: https://www.photrio.com/forum/forums/alt-print-exchange.42/
* Blind Print Exchange. You'd have to check the posts in the relevant sub-forum (https://www.photrio.com/forum/forums/blind-print-exchange.44/), but I think digital prints are excluded.
So the postcard exchange is the logical place to start.
Alternatively, you could set up an exchange of your own, but I'd suggest starting with the postcard exchange first to get a feeling for how the practicalities of an exchange work, and then maybe set up something new if you feel it would add something to the party.