More like the bus version of Uber, they don't own the buses (well, since they've just bought Greyhound, maybe they do now) but instead rely on "partner" companies to own, maintain and drive the buses.
Perhaps partners are how they established a foothold in America?
Take a careful look at the front side of the bus, driver side, and the front door. There will be the logo of the partner operating the route.
Flixbus and the various companies they bought up over the years requires the partners to decorate their buses in the Flixbus CI (green/orange + logo). Exceptions apply for peak-capacity / spare buses, these however have to be decorated with a magnetically attachable Flixbus logo if in service for Flixbus.
If you have your journeys saved on the mobile app, they also say "Operated by [partner name]".
It's not unusual to see an all-bus (or all-train) wrap with advertising, or the logo of the bus (or train) company.
That is one of the differences between Uber and FlixBus, the latter actually respects regulations. They doing so in creative ways, but they don't ignore them.
Sucks for the bus partners, but FlixBus didn’t even have any layoffs, at least not in Germany.
(Inside info, I can tell you that FlixBus has been mighty proud of their ability to scale up and down their networks during the pandemic and at least the internally communicated credo has always been that while this is all hard and challenging they might even leave the pandemic strengthened and in stronger position than their competitors, something this acquisition also hints at.)