Most roaming is handled through 3rd party companies who handle all of that - from actual data transport to billing. The telcos themselves generally won't bother maintaining specific infrastructure with other (foreign) telcos as the overhead is simply too high, so they instead use these clearing houses. I'm sure there are exceptions (eg, AT&T and BT probably have enough cross-over to have something direct between them), but most roaming is handled outside of the telco itself.

Whilst I can't say that Airalo aren't an MVNO in some counties, they clearly aren't in general. The fact that their country-specific eSIMs route through random countries that are (generally) not the country you've visiting makes it clear they are just reselling others eSIMs. If all of their Asian eSIMs routed through Singapore (where they are based) or some other single country then I'd think differently, but at least in my experience that's not how they work. (Their Japanese eSIM does route through Singapore, but that's the only one I've found that does)