Tracfone, Net10, StraightTalk are programs through Tracfone to re-sell service on various GSM or CDMA (depends on where you want to activate) providers. Used to be the GSM coverage was more complete (multiple providers) but CDMA is a lot better now if activated as Verizon.
Tracfone works for me because it costs me about $6 a month to keep the phine active, and I'm still buying more minutes than I'll ever use. But data has to convert units (minutes) back to dollars based on how I bought them, and get charged by dollar per data quantity. All bookkeeping is done on the phone itself. Net10 is also phone accounting, structured so you pay a fixed amount per minute, rather than buying blocks of activation time.
TracFone and Net10 are almost giving away the phones, so if you buy a cheap enough phone it can be treated as disposable.
StraightTalk is sold through Walmart, as so much per month unlimited text, talk, and messaging, and a fixed amount of data at high speed before throttling. I think the standard plan includes international calling, something you buy as an separate service for Tracfone. You usually pay close to full price for smartphones (or bring your own maybe) but the simple phones are fairly deeply discounted.
Virgin resells Sprint services. Data devices and phones are separate programs. The difference between buying as Virgin versus contracting Sprint is that Virgin stays on the Sprint network, Sprint customers can roam to some other compatble networks. Cricket is a similar reseller, usually on Sprint, maybe always.
Millenicom resells data services, on Verizon and Sprint networks, you plan is one or the other, not both, different terms.
All the major providers, ATT, Verizon, Sprint, T-Mobile have their own pay as you go plans, in most cases data devices and phones separately. You either pay for blocks of time/data, or tiered levels of service per month. Rates are often twice as high as resellers. International calling varies.
Look into plan details for roaming, some pay as you go will roam into Canada or Canada and Mexico, or even some places in the Caribbean, which others are continental U.S. only, depending on provider and reseller arrangements with provider. I don't know of any Prepaid with global roaming,