Since the rules haven’t gone into effect yet I wouldn’t worry about it, but if/when they do, I would make sure to mark all transactions with notes describing what they’re for, so you can sort them later.

Then if you want you can print out the receipts but I’m not really sure that would be necessary unless it specifies on your tax forms that you need to. I’d have to look at mine to see and I don’t have them on hand.

They really should only require strict itemization for auditing purposes, but I’m not an accountant or even in a tangentially financial field so don’t take my word for it.

Probably a record of receipts from whatever vendor you have. You should have a transfer history in venmo/PayPal/cashapp that will show you who paid you, how much they paid you, and vice versa. Not sure what else you’d be able to produce.