Gift tax and question re: sharing an account with someone who is not your spouse

I am going to pose these questions in a largely hypothetical manner because I don’t want to share personal stories about my family on the internet, even if it is anonymously.

If a relative gave me a sum of money and I used it to open an online brokerage account, what are the tax consequences? Specifically:

Because I could withdraw every penny if I wanted to, does the relative have an obligation to report this to the IRS for gift tax reporting purposes? Also, if there were any dividends or interest earned or capital gains realized in this account, on whose tax return should these be reported? The relative’s or mine? (I file jointly with my spouse.)

Thank you for any enlightenment.