Pardoning federal prisoners, Biden can do all on his own, and already did. (Didn’t do all that much because more people are in state prison for state charges. But, if you’re one of those people who’s now out of prison, it’s pretty significant I think.)
Rescheduling marijuana takes agreement from the DEA, who aren’t exactly weed-radicals, and the value of doing it is a little limited in the first place. He requested to reschedule it years ago and they’ve been dragging their feet on it for whatever reason up until very very recently.
A bill for decriminalization is where the real significant change can come. There were a couple of them that came along, of which probably the most serious effort was a bill for full legalization. It passed the house, but there was a little bit of Democratic opposition when it reached the senate, and of course all the Republicans voted against it, so it failed. So, quite literally, 50 people would have been needed in order to pass it, but we couldn’t get the 50 together, which is why it’s still federally illegal.
It actually does take a lot more than one person.