That's precisely what the phase 3 vaccine trials are doing - by large sample size vaccine trials in communities where C-19 is in circulation, and by vaccinating volunteers in smaller scale trials, then deliberately infecting them. The latter sounds great (brave move) but has a flaw that for ethical reasons volunteers exclude those most vulnerable to C-19, so you're not going to know for sure immune response in elderly etc.
It's premature to say that there's definitely some (cross) "immunity" because some people have different symptoms - the same could be said about smallpox or basically any other viral infection - some die, some don't etc. We're all different.
Do you have a source for human challenge trials? I have certainly read nothing about these being undertaken anywhere, and Google can't link me to anything either. I had read stories in recent weeks about the potential for this (and some people pushing for this), but infecting people raises all sorts of huge ethics issues hence the fact it's vary rare.
The phase 3 trials that are occuring are largely being tested in Covid-19 hotspots where community transmission is occuring, and testing and placebo testing with people such as doctors in hospitals treating patients. This is very different from purposely infecting somebody.
Jenner Institute / "Oxford Vaccine" is gunning for it.
(Jenner - LOL - grabbed his gardener's 8YO son for a "challenge trial" for smallpox - could have been hero or zero, was hero fortunately for humanity and the 8YO boy).