I’m no expert but I believe that in general readers that accept multiple files actually read them in as an explicit list. The wildcard/family selection mechanism is only implemented in the file dialog, but the wildcard is expanded to an explicit list before being passed to the reader.
For example, the XDMFReader Python signature is
reader = XDMFReader(registrationName='name', FileNames=['/path/to/file_001.xmf2', '/path/to/file_002.xmf2', '/path/to/file_003.xmf2'])
The XDMF format has a way to define time for each file. If a format does not (or if the times are not defined) then I think each file is given an arbitrary time of 1s,2s,3s, etc. If a format does not support time, then you can create a wrapper meta-file where you define times for each file.
Check out https://discourse.paraview.org/t/assign-time-to-a-timestep/3505
And yes I believe that in general readers are written for an individual file, and the vtkFileSeriesReader is used as a manager to hold all the file names and pass them to the reader as requested.