Arp works similar to monophonic synths, it cycles through current notes while they are playing.
In your example: if trigger length is say 24, then arpeggiator would start at step one. on step two it would cycle between notes on triggers one and two (triggers can be chords). And on step three to 24 it would cycle notes on triggers 1-3. And after 26 it would stop. Lenght of the arp pattern (whatevervitnis called) doesnt affect how long the arp is playing.
Things get messier though when you introduce soundlocks that have arpeggiator… and one propably could achieve what you are describing
Tested it. Actually it doesnt with sound lock either. A new soundlock with arpeggiator cuts out any arpeggiated notes that came before. So for arpeggios to overlap you would need to use separate tracks.
Trigger length = how long a trigger plays. Ie a trigger on step 1 with length 4 would keep playing until step 5.