It's set to loop by default. No change. Thanks for trying.

What you're looking for is "Hold". There is more than one way to turn this on.

First way - from [PERFORMANCE] (HOME) - the home screen

1) Press [SHIFT] and touch the "Arp On" (or "Arp Off" first to turn it on so it's blue) square on the Part you want to hold. When pressing [SHIFT] + "Arp On" the blue square turns orange and now the Part has hold turned on

Second way - from Part's arpeggio configuration.

... navigate to the arpeggio common settings and turn Hold ON

You notice there are multiple settings - "Sync Off, Off, and On". Turn it On.

A third way - change how the ARP is triggered altogether.

Although not a "hold" - if you change the Arp's trigger from gated to toggle - you'll notice the behavior is similar to a hold - but you can cancel the hold with another trigger. Like an "on/off" switch - not just an "on" switch.

1) Select the Part, edit it. Make sure the "Common" blue square at the bottom is selected.
2) Go to menu "Arpeggio" -> "Advanced"
3) "Trigger Mode" change from "Gate" to "Toggle"

Now the arpeggio continues to play on its own when you trigger it until you press any piano key (within the trigger range) to stop the ARP. Trigger range is not limited to the notes you used to start the Arp - but all of the notes within the range shown under "Arpeggio" -> "Common"'s "Note Limit" range. This note limit here are the range of notes that trigger (or stop - for toggle) the arpeggio.

That's correct. Just like you cannot have some arpeggios in the same Part hold and some not hold. These are all Part-level configurations.

The common way to mimic starting and stopping a hold is to hold all arpeggios but use "Mute 4/4" (if that's your time signature) to "silence" the note output. So changing arpeggio numbers would invoke the Mute arpeggio instead of cancelling hold.