Is this a per-contact setting or a "universal" one?
Each whatsapp user needs to enable the setting once to allow chats with multiple number of third party users.
Account-wide. Though you can only turn it on in Europe.
When you say Europe you mean the EU? I'm not seeing an option in the UK. (Yay Brexit)
Yeah it's the EU, this is a result of the DMA regulation. I suppose the British government could enact a similar regulation tho.
It's a universal setting. You have to enable it per third-party app, though. You get to choose whether you want to see them listed with WhatsApp chats or in a separate folder
It's universal, but you need to whitelist specific apps people can message you from. This is what it looks like: https://i.imgur.com/0gKY76z.png