They largely seem to indeed be reselling the services of a few wholesale providers, in my experience.
Some of them literally just resell local SIMs on a different storefront, but that seems to be the absolute minority.
Most of them sell eSIM profiles not "based" in the country you're actually buying data for, which I suspect allows them to effectively provide access to, and often arbitrage of, the rate carriers in the visited country would charge their users directly.
I've experienced all kinds of interesting combinations, such as receiving an ICCID from country A, IMSI and (usually) IP address from country B, while traveling to country C. (In this case, the reseller pays the provider in country B for access to their wholesale roaming rates between the visited network in C and themselves, I assume.)
Some providers actually use Multi-IMSI SIMs under the hood, which is yet another level of virtualization, allowing them to dynamically select a "home" network closer to your visited location, improving latency and the distance your data has to travel. (Truphone, now BetterRoaming, does that, for example.)
Some of them literally just resell local SIMs on a different storefront, but that seems to be the absolute minority.
Most of them sell eSIM profiles not "based" in the country you're actually buying data for, which I suspect allows them to effectively provide access to, and often arbitrage of, the rate carriers in the visited country would charge their users directly.
I've experienced all kinds of interesting combinations, such as receiving an ICCID from country A, IMSI and (usually) IP address from country B, while traveling to country C. (In this case, the reseller pays the provider in country B for access to their wholesale roaming rates between the visited network in C and themselves, I assume.)
Some providers actually use Multi-IMSI SIMs under the hood, which is yet another level of virtualization, allowing them to dynamically select a "home" network closer to your visited location, improving latency and the distance your data has to travel. (Truphone, now BetterRoaming, does that, for example.)