My 2012 Mazda3 has an external amp under the front passenger seat. I bought my Pioneer radio from Crutchfield and they included an adapter to make everything work. The amp works fine (most radios have a preamp output for this purpose), and the steering wheel controls work fine too.
My car just had a basic radio though. I guess it might be harder in newer cars with fancier radios. People that mount their phone or use things like this Spotify… thing usually have older cars/radios though. The newer ones just have Android Auto and Apple Carplay built in.