how to make peanut butter not grainy

There's only so much you can do with an ordinary home blender or food processor. A more powerful machine will give better results than a less powerful one, but only to a point.

You're just not going to match the smoothness of commercial peanut peanut butter, which is produced by extremely powerful and expensive grinders with close tolerances that can grind the peanuts to a smooth paste.

You could put the peanut butter in a food processor and add vegetable oil (a little at a time) while running the machine.