Can my ISP track my activity in phone apps like Reddit?

To keep it simple, your ISP can see the URLs which will show them the subreddits you're clicking on, but they won't know what you're actually posting unless it's public. That's thanks to HTTPS! Still, they don't have access to your private messages or comments.

If you're really curious, tools like Wireshark can show you how data is being transmitted. The Reddit app does use encryption, so your ISP can see the URLs but not the specific content of your posts or comments. Just a heads up that most ISPs do use data tracking for ads, though!