What happens if I "join" a presidential candidate s campaign?

Your name and contact information will go into their list of supporters. They will then use that information to contact you with further solicitations for money, to volunteer, and to get out and vote. They will also likely use that information for some of their data modelling. They know that you liked them enough to fill out a form and go on a list so they will use that information to help plan their campaign. With most of the Democrats they may also share that information with the DNC and with other candidates if they don't end up winning the nomination. Sanders might be different though. He's only a Democrat as a flag of convenience. IIRC back in 2016 he refused to share his sizable donor list with Democrats.

You will get bombarded with e-mails and phone calls asking you for money and you'll never hear the last of it.

They'll spam you with emails to donate money. If they don't get the nomination, then they'll give your email address to the party nominee who'll also spam you with emails to donate money. And they'll give your address to the DNC who'll also spam you with emails to donate money.

They will ask you for financial support.

Beware...you do that and you'll be bombarded with mail and email asking for donations. And if you do give, you'll bring on even more mail and email. It's never ending.

Be warned.