Mostly they are matched to two classes of tow bars.
Roadmaster attaches towing brackets permanently to the car, then attaches a removable baseplate (which comes with the tow bar) to those brackets.
BlueOx, Demco, some older manufacturers no longer prominent in the retail towbar business, permanently attach a baseplate to the car, providing "tabs" to which the towbar is attached. There are differences in tab configurations, but there are also adapters. Other towbar makers like NightShift Auto (ReadyBrute brand) will attach to a Blue Ox or Demco baseplate.
Things are starting the change, as some modern cars don't have the front end strength to handle two independent towing brackets, so the Roadmaster towing brackets for this will include hidden baseplate. These models often replace the cars hidden bumper. The external baseplate or cross-bar is still attached before hooking up the towbar.
You can do adaptations. You can buy an external baseplate to attach certain BlueOx towbars to Roadmaster brackets. You can buy fittings to install the external crossbar/baseplate from Roadmaster to the tabs coming out from a BlueOx permanent baseplate. But you can't necessarily adapt anything to anything.
The cleanest looking (if you are looking for "invisible") installation is made by using brackets or baseplate from the manufacturer of the towbar, or at least of the same type of towbar connection.
I chose my type of towbar based on the instructions for fitting of brackets or baseplate. For my Ford Ranger, I chose the Demco baseplate for how it attached to the frame horns and where the tabs came out, the others were much lower for that installation. For my Honda Fit, I chose the BlueOx baseplate, because of how it attached and because when all put together, I still had my original bumper. The Roadmaster bracket/baseplate solution for that car and model year meant discarding Honda's impact-absorbing bumper.
So with any given car, not only will the fittings be different for different towbar brands, but the installation might be quite different for various baseplate/bracket brands.
Tom Test
Itasca Spirit 29B