It depends on the project. Often there’s a “Contributing” page on its website or a file named CONTRIBUTING.md or similar in the source tree. Start from reading it. Sometimes information there is aimed at experienced developers, but sometimes it is very friendly for beginners.

$ sudo smartctl -A /dev/sda -f brief | grep -i power_on
  9 Power_On_Hours          -O--CK   001   001   000    -    120198
$

If you don’t need issue tracker, wki, CI etc., cgit will be enough. If choose from gitlab, gitea and forgejo, use forgejo.

Do you really need OPNsense? Buying a OpenWrt capable router would save your money, place and silence.