do you have to make a character for that campaign or can you use one you already have?
You can do either. The DM will give you a link to join the campaign; when you go to that link it’ll let you select one of your existing characters or make a new one.
does the player have to pay and have a subscription too?
No. Only the DM needs to have a subscription to use content sharing.
pronouns: he/she/they
so if i get to join a said DM then i can play in the DM's campaign without having to buy a Beyond Subscription?
Correct.
pronouns: he/she/they
It doesn’t even have to be the DM, I believe. As long as someone has a master-tier subscription, everyone can share everything.
do you have to make a character for that campaign or can you use one you already have?
You can do either. The DM will give you a link to join the campaign; when you go to that link it’ll let you select one of your existing characters or make a new one.
pronouns: he/she/they
nice
I find that if I try to make a character outside of the campaign, I do not get the benefit of shared content
Yes, that's how it works. Content is only shared within the specific campaign.