Iām no Youtuber so I canāt tell you what software/hardware is needed for video recording or streaming , but on the audio-side I may have some suggestions.
Connecting SP-404 to computer:
From my experience the USB-audio works pretty much āplug & playā. I did this with iPhone, iPad and PC, so I expect this working with Mac as well. This said, you donāt need an audio-interface. An USB to USB connction should be enough to start. Cable = USB-C to USB-C.
Connecting SP-404 to iPad/iPhone:
This takes the so called USB adapter. Advantage is that you can connect to the power supply as well. My experience is that many iOS apps for audio production have quite a high toll on the battery. So charging and using might be interesting for you
Audio Interface:
If you want to record the audio from the line-outs of the SP-404 you should buy an audio-interface, because the AC/DC and DC/AC convertion is better done outside a computer. Most processors are just not made for heavy loaded audio tasks. If you donāt need too much inputs/outputs and want a decent monitor/headphone output as well, get a simple audio-interface with just those inputs and outputs.
Check out as an example the small Focusrite series or iConnectAudio interfaces. Typically the audio quality is studio worthy. More expensive interfaces just have more channels and a couple of bells and whistles you might not need for the beginning.
iConnect interfaces might be interesting, because they come with some features supporting the Apple universe quite well. I use a iConnectAudio for having 2xaudio in, 4xaudio out, MIDI in and out and additionally connectivity of a iPad over USB in a small box.
iOS apps:
On an iPad I would suggest to check out AUM. Thats a mixing desk like app, which allows for all kind of sound-generators, synths, sequencers and external gear to be connected in a matrix like fashion and itās very flexible. Itās like having a patch-bay inside the iPad ā¦ if that makes sense to you
There are so many iOS apps for creating music that I canāt tell, what would be interesting for you. There are synths, samplers, groove-box like apps, and all can be easyly connected inside AUM and the audio can go in/out via USB-audio to any device outside the box.
Recording on the Computer: what about Garage Band, Logic and many others ā¦
Recording on the iPad: Garage Band, Cubasis 2 and maybe some others ā¦