Disclaimer: I do not watch vtubers.

Is this like a real person that uses software to make them look animated, or is it a company that designs a character and hires a voice actor

It's a combination of these two. An actor (often already a streamer/entertainer of some sort) is hired by a company (there are also independent vtubers but Pikamee isn't one of those). The actor and company design a character for them to play on their streams. In theory, nobody knows who the actor is, though IIRC many have been figured out. Even so, most people ignore this, and only refer to the character.

And if enough people are mean to them in the comments, they stop making videos forever and say that they “graduated”?

"Graduating" is slang meaning they quit. This could be due to anything because the companies don't say what happens.

Could the person behind Pikamee create a new character and continue making videos under a new persona

It's possible they might have some non-compete agreement or something so maybe, maybe not. Also note that there is substantial cost in getting a decent avatar (several thousand dollars).

could a different person make Pikamee videos?

Possible but unlikely. I think this happened once for one of the oldest vtubers and people flipped out (it's worth noting that vtuber fans are extremely online). Much more likely to just make a new character instead.