I wonder how to go about to collaborate to the OSM ecosystem with code. Does anyone know what are the cool projects within OSM in C/C++? (i.e. not web stuff)
Take a look at Tippecanoe, which is under active development again[0]. The original developer, Erica Fischer (who is wonderful to work with), has a fork[1] where new work is happening.
(Co-founder of Felt here) Thanks for the kind words! We are lucky to have Erica on our team. If you ever want to talk maps, my email is can@felt dot com!
The biggest help one can probably be to the project is to create or improve user-facing software. OsmAnd is still clunky and ugly to the average person coming from FAANG Maps, for example. The iOS version is even worse. Some alternative apps exist with better UX, but they're often missing features. There are also some web projects out there that aim to be a google maps website replacement, but none of them are quite there.
It could also be a niche thing, like if you make an app that shows the nearest defibrillator or other emergency facilities that people might need in a pinch. Any area gaining good coverage of this type of data would immediately have an app to turn to. With adoption also comes the influx of new contributors.
I use Maps.me as my main map and navigation app as I don't like that Google knows where I go, and I can say that it's a very good alternative to Google Maps. However I have to say that now I'm a little worried because it's Russian-owned. Happy to know that there's an open-source alternative.
The most accessible general-purpose map app for end users (Android and iOS) is written in C/C++. This is pretty high on the cool and useful scale: https://organicmaps.app/
[0] https://felt.com/blog/erica-fischer-tippecanoe-at-felt
[1] https://github.com/felt/tippecanoe