Then we have a match: postmarketOS is the type of project that happily accepts code contributions ;)

Here is the big overview link of all sorts of things where people can contribute to: https://wiki.postmarketos.org/wiki/Contributing

I suggest to familiarize yourself with the project first. That is, play around with it in qemu, flash it to a device if you have one or start your own port (porting is quite feasible, as one can see from the steadily increasing count of booting devices). Then work on what interests you most, and if you need some inspiration, take a look at the gitlab issues that are tagged with "help wanted". (Some are also tagged with "easy", which is obviously a good start.)