How does Infuse Pro play everything in direct play from a Plex server?

Curious. I think it is truly great!

Because it tells the server that is able to play (nearly) everything and doesn’t put limits on steaming.

TBH even the (new) official Plex client is able to do the same so transcoding on ATV is a thing of the past.

Still for some “heavy “ files Infuse still has an edge and plays smoother

Not true. Plex transcodes HD audio to FLAC.