I would need to measure and create WiFi coverage heat-map for an area with multiple APs. - What software would be best for doing that on a Mac or Windows notebook?
I prefer Mac, but can use wither PC laptop or VM for testing.

Thanks

Give inSSIDer a try.

Is there some kind of trial? I don't have a clue what to expect from it? Any tips?

Did you at least have a look at the site? There is a link to free "lite" version: https://www.metageek.com/products/inssider/free/
If you want to see history of channel strength but don't want to pay, you can also go for old "home" version which was also free in the past and can be still downloaded via unofficial channels: https://www.techspot.com/downloads/5936-inssider.html

Thank you; I really haven't find the free version before, tho I did online. I haven't been sure about it, thus I asked.

Decided for NetSpot in the end. Worked well.