aosp system service

I've been developing a custom aosp ROM, and I'm not familiar enough with the aosp code base to create a system service (or hijack an existing one). Because this is for a custom phone, none of the changes will be going back upstream to Google, and therefore changing the code for aosp itself is allowable. i'm needing to make a system service that I can put regular scheduled tasks into for my product (a custom phone), such as making http calls to a rest api and synchronizing an sqlite file on the phone with data from the rest calls. Because Google probably doesn't want anyone changing the aosp code based, there isn't a lot of documentation on how to do this. A few articles online exist, but they can be misleading sometimes when written for older aosp versions (we are on aosp 12). I'm also looking for someone that has worked in the aosp codebase for more projects going forward, and am testing the waters to see what this would cost, especially when faced with situations that i don't know how to do things in aosp, and could turn to someone more experienced.