Was watching this speedrun and at the beginning they just die and respawn to the pianta point, how? Speedrun:

from da FAQ:

What does NG+ mean / How do you warp to somewhere else in the castle by falling out of bounds? NewGame+ categories involve use of a glitch called Respawn Bug Abuse to reach areas of the castle early on a new file by storing the respawn point of the last level you entered on a different file beforehand. In its simplest form, this involves entering a level on a completed file, such as Bowser in the Sky or Lethal Lava Land, and then exiting to menu. Once you start a new file, you can simply exit to castle and then void out (by clipping out of bounds next to the JRB portait, for example) OR die by taking enough damage (spam P+V) to be taken to the entrance of the level you had exited from previously.