Fundamentally, nothing really. It's all the same: money moves from one person to another—quickly, instantly, and no fees (unless it's a business transaction)—as long as it doesn't leave the network. But therein lies the rub: money has to leave this network occasionally because you cannot send money peer-to-peer to from PayPal to a merchant not using PayPal (like Amazon), and that is the troublesome part where money has to be ACH'd to bank accounts.

And as such, the p2p ecosystems differentiate themselves by giving people ways to pay merchants without explicitly needing to move that money out of the network. So, Cash App issued a debit card, and then PayPal, and then Venmo (yep, even though it's a subsidiary, it's a separate card). And then some of them started using real bank accounts (with routing/account numbers) to back the stored balance. And the result today is that all these p2p apps look awfully a lot like banks, but less encumbered by regulations because they don't do everything that banks do.

Back to your question, why bother changing to Venmo (or Cash App) when PayPal is basically the same at this point? Obviously, the primary reason would be if everyone you know is using not-PayPal. A secondary reason is the same reason that people have different credit cards from different banks: different rewards schemes (yep, there are incentives for using those p2p debit cards).

Zelle is probably the odd one out here, because it's just an extra thing built on top of existing participating banks that already have all these ways of moving money out of their networks. It's effectively a wire transfer replacement without the fee (or a physical check replacement without the check-writing).

(Disclosure: am employed by a company that has a p2p product.)

PayPal have an unfortunate history of seizing people's balances. So it's fine for small amounts, but you shouldn't trust it for large amounts if you can help it.

PayPal is complicated and difficult to use. There are lots of different ways to pay people and they use dark patterns to try and get you to use a method that costs you fees. And this is coming from someone who has used it for 20 years. Venmo is starting down that same road, likely due to it being purchased by PayPal.

To pay with PayPal, cash moves: Your Bank Account -> Your PayPal Account -> Their PayPal Account -> Their Bank Account.

That's how a client-server model works, definitely not p2p.

I have no clue how Venmo or Zelle work though, so I don't have an answer to your latter question.