How does a single-SIM MVNO on mulitiple networks work?

So if you get straight talk, you have to select from 4 SIM cards to pop in, and that activates your phone on that particular network... I get that. Lately, however, I've seen multi-network MVNOs arise (like Defense Mobile, ROK Mobile) that only use one SIM card. How does a single sim card authenticate between several carriers? How does a single sim multi-network MVNO handle that extra step Verizon usually requires of authenticating the individual phone's IMEI with it?

Please help me understand.
Thank you.