I count it if I step foot in the country (airport counts), though ideally I’d like to leave the airport and see each country. It’s a personal thing, so others may have a different opinion here. I counted HK here because I had a layover there, but I’ve been to HKG a couple times before (spent several days there) so it wasn’t a new country for me. In general, if I only transit a country, I typically plan to come back another time to enter and see the country a bit more.

And I do count day trips. Some countries are very small and don’t really need an overnight stay to see the sights. Gibraltar, Macau and Brunei come to mind.

That said, 30 countries by 30 should be a doable goal. Europe and SE Asia are regions that are very conducive to seeing a lot of countries in a relatively short time.