All sub-panels are, as the name implies, a "sub" off a "main". So the configuration is a breaker in the main panel that feeds the sub-panel. Being more specific, the 40 or 50 amp breaker goes in your main panel. Then HEAVY cable from the main panel to the sub-panel (separate discussion on size of cable). This 40/50 amp feed acts as the main feed for the sub-panel. Each breaker in the sub-panel draws from this single line to the main panel, which in our case is a single feed and thus all on the desired same phase.
@DaveC951 - I think I understand the importance of the whole system only on one phase because of the phase shift between phases, but is this also effective with extreme powerful mono amps like for example EMM Labs MTRX or MTRX2? Wouldn't it be better to split the amps on two phases because of the heavy load?
And regarding size of size of heavy cable from main to sub-panel or also the size for dedicated lines especially for powerful mono amps is there already a separate discussion on the forum?