Roaming nowadays has largely been surplanted by E-sims, but in this scene they're talking about inter-state roaming charges. The woman in the screenshot is from florida and she's supposedly getting roaming charges while in Pennsylvania
Even when the show came out roaming wasn’t a thing. Roaming has nothing to do with esims and it has to do with which cell network you are connected to. In the American context, if your provider is Verizon but where you are you can only connect to T-Mobile towers then you are roaming.
Some of a telco’s biggest customers and biggest vendors are other telcos.
MVNOs operate on MNO networks. MVNOs have agreements on access and usage to use MNO networks. MNOs also have agreements with other MNOs to roam on each other’s networks. These are called access charges and access revenue
It's kinda dumb when all this could just be nationalized and run under one umbrella, but MNO and MVNO leasing is at least better than having to pay roaming charges because Verizon or AT&T don't have ownership of the Bell infrastructure in your region because that's how the cards fell when they broke it up.