mm wrote:

What do I have to do to clean an electric oven after I have sprayed it
with a fire extinguisher, one that sprays the white powder?

If I miss any of it and gets in my food, is it poisonous?

It's a "continous cleaning" oven, that has some sort of porous coating
that absorbs grease and brings it graduatly to the surface to burn
off.

Arm & Hammer used to make a non-caustic oven cleaner that worked by
heating the oven to 450F+ for 30 minutes. I think they sold it to
Easy-Off, and it's now called Easy-Off Non-Fuming.