rerouting soundjack output to zoom

Sorry for not having a solution yet. I tried it on my Mac with a Jack audio buffer of 128 and had problems with sound quality. Then I directly connected the interface to zoom via Jack and that gave me a peak visual in Zoom at least.
I think it might have to do with the sound buffer size, since the buffer sizes (and sampling rates) of all applications and sound hardware have to be the same. In Zoom I didn’t find a selection for the sound buffer size, so You might need to experiment trying different network buffer sizes for SoundJack and the same buffer size for Jack.

Not a solution, but maybe that helps you trying to solve your problem. I might check again during the weekend (and maybe even on a Windows 10 machine). Don’t forget, that the sound input and output on a Windows machine needs to use the same driver, in this case Jack.

Dividing the apps onto two machines is a good idea. It keeps the load for the soundjack computer as low as possible.
It also makes sense not (!) to use the same network for soundjack and zoom (or whatever you want to use for distribution).

This is also explained in the very good video from a sax player and an accordeonist (Duo Aliada) that has been on the start page of soundjack for a while (some time ago) and is now on the startpage of the fastmusic tab on symonics.com website. It is also directly available on youtube:

You need to see the whole video to understand what they have done to realise it. They tricked a lot for the video.
The streaming part starts at 18:35 for about three minutes to the end.

The use of a mobile device for distribution gives you the choice to use a mobile connection, but you can also check if a WLAN connection would be acceptable. Just check how you want to get the sound into the mobile device, an audio interface might be needed or you need to adapt the higher line or headphone out to the sensible mic input by using an impedance matching transformer. (But you can also try a two resistor voltage divider to reduce the volume voltage.)

If you have another computer, you can use a mobile phone with tethering or a mobile data USB stick to make the mobile connection and run the broadcasting software on the computer. You might need a second audio interface if this computer doesn’t have a line in connection.

Using localhost for local mirroring should work.
What I found out is that the dropouts/crackling (at least on slower machines) mostly come from other processes running on the same machine, so that the sending or receiving of network packets gets delayed. The worse the more load the CPU gets. So even with localhost some dropouts/crackling can happen if other web pages are open or background processes of other installed software gets active.

The main out of an audio interface is typically (consumer) line level, about 500 mV max, around 600 Ohms output impedance, also described as -10 dbu. They are intended to drive an input of an impedance of 10 KOhms or higher.
Microphones are in a small mV range, depends on the type and max. SPL. I assume around 1 to 20 mV max, but with an output impedance of 200Ohms to about 1 KOhms.

As far as I know the input impedance of the iPad/iPhone is 1 KOhms. So it needs to be driven with an impedance which is a lot lower, probably around 200 to 500 Ohms. You wont get that from a line level output. A transformer can reduce the voltage of the line level output and at the same time reduce the impedance too, so that would be the way to go, since putting 700 mV to a 10 mV sensitive input would greatly overdrive it and may even harm the circuit. But there are adapters for iPhone and iPad which enable an instrument (level) to be connected to such a mobile device. That may work with the line output, even though instrument level can get a bit larger than line level. The main difference of an instrument input to line level input is that the impedance for an instrument level input is much higher, typically around several hundred KOhms to 1 MegOhms. That would work impedance wise feeding a line level signal into an instrument level input if no line level input is available. It might be a bit quieter than optimal but should work.

BTW, the input connector of an iPad/iPhone has four connectors, ground, left, right and mic. The mic of the headset is an electret condenser which needs supply voltage which is fed on top of the mic signal level (via a 1 KOhms resistor, so the mic signal can be added to it, in german it is called “Tonaderspeisung”), so you need to strip that off by a capacitor in series (breaking a direct hot connection). If you don’t know what to do, better get an adapter from a store or an audio interface for the iPad/iPhone. Some iPad/iPhone mics have a line in too which disable the mic if used. That would be optimal and hopefully not too expensive.

I just wanted to explain that things like that are a bit more complicated than making a purely wired adapter cable.
I think the second computer with its own audio interface and a tethered mobile internet connection is easier to put together and cheaper if you use a cheap Behringer interface as a second one.

That simple question did get a bit technically, I know. Because it is not as simple as it seems to be.

if you want to use a DAW too, there’s another app in the middle called OBS, which is what he’s doing in the video.