> PS. does anyone know what the purpose of config.json is, since changes here are not reflected, and since I managed to compile a perfectly working (except push notifications) customized app without touching src/config.json?

The config.json file is the one you should be modifying, there is a gulp task that generates src/configconstants.ts reading this json file. If you place your modifications in the typescript file you risk losing them, because that file is not tracked in the repository and it can be overwritten if some script regenerates it.

You can regenerate it yourself by calling the gulp task named config, for example doing npx gulp.