I know the borders are closed for the purposes of Americans entering Europe or Canada for example. But would it theoretically be possible to say, fly EWR-MUC and immediately turn around and do MUC-EWR? Or EWR-YVR-EWR?

If you stay airside, is it allowed by United (Germany for example states that travel to Germany is allowed for Passengers in Transit.)

Please leave judgements aside as to whether this is a good idea during the pandemic, I'm looking at what will technically be allowed.