Explain SA missiles to me.

So I get that SA means I need to be pointing at the target until the missile hits, nice, Fire and forget, I can turn around, nice.

My question is, how is it that when I use an SU-25 an SU-25T, i can just move them around and missiles seem to hit even if they don't exactly aim at the target ? Is recon guiding the missile, is any friendly unit able to guide an SA missile, no matter if the unit didn't fire the missile ? I don't get it....