You could still get everything working by manually configuring the phone, and carriers like GiffGaff had support articles explaining how, but it was a total pain in the arse and stuff would occasionally break in subtle ways.

Additionally the eSIM is separate physical chip within the phone attached to baseband radio. Once configured with a profile, the host OS has as much access as it does to a normal SIM, I.e. not much. The entire standard is kinda a hack, which basically boils down to “what if we embedded a normal SIM card chip in the phone, and let the phone flash a new carrier profile every now and then?”. Remember these SIMs are fully fledged computers, like their normal removable counterparts, that can ignore host OS commands, and issue instructions directly to the baseband processor without the host OS knowledge.