The Arc will only connect over 2.4GHz. But the Arc will communicate wirelessly with the Subs and Ones over 5GHz.

In general, 2.4 GHz has greater distance and better penetration of materials, so you’re better off using 2.4Ghz. As suggested, Sonos does use a ‘private’ 5Ghz signal between the sound bars and the surround speakers. 

'Private' 5ghz signal to the subs as well?

'Private' 5ghz signal for the subs as well?

Yes. Since the 2.4Ghz system is in use to connect to the router, the 5Ghz channel is used to connect to both the surrounds and Sub. 

SONOS uses 5GHz for surround/sub communication because 5GHz is usually faster and the surround/sub is very close to the bar. There is no point placing the sub/surround down the hall and around the corner from the bar. This can get a little dicey if a wireless AMP is used for the surround speakers because AMP might sit down the hall and around the corner while wired to the surround speakers.