The way I work is using the 404 as a sound card. I use koala either on my iPad or my MacBook (it works exactly the same) I can either run my turntable in to my SP and record samples straight into koala, or record them into the SP, apply effects, chop, etc, then run them in.
If you don’t have a turntable or want to sample YouTube or any other audio, you can do a screen grab of a video and koala will import it as audio. I sample like that often. Then you have the audio in the app and you can run it back and forth between it and the SP. I do most of the heavy lifting in koala and use the SP for its effects.
I’ve also used Serato studio and the 404 combo lately (Serato studio in place of koala) and it is also a super dope combo.
The best thing to me about koala is that I’ve started beats on my phone, used airdrop to finish them on my laptop, used the feature in koala to save as an ableton file and mixed everything easily. If you told me 10 years ago I would be doing this I wouldn’t have believed you. It’s so seamless
I’m pretty sure you can transfer files using the 404 app but I’ve never done it.