How is this getting upvoted, zram is a ram compression technique of a few % and has nothing to do with preloading strategies
Edit: mb, apparently its behaviour now also includes general purpose cache. Back in the day, I remembered that it would only help for low ram devices to make better usage of existing ram.
This happens on most Android phones, including my Google Pixel with 8gb of RAM. I noticed that it starts doing this when battery drops below 50%. Which is weird because it surely takes more resources (and thus drains more battery) to cold start an app every time instead of just keeping it in memory (provided that app doesn't do anything nefarious in background which is easy to detect). There is plenty of RAM for that.