If you have production plants in your inventory, they are used automatically during the creation of your corps. I can't remember if it's one or two production plants per created corp. If you don't own any production plants, they are purchased automatically.
I don't believe there is any benefit in using production plants of a quality higher than 120. For purposes of corp creation, higher quality production plants seem like a scam.
Since their price fluctuates a lot, you can buy them when they are cheap and sell them later for a sweet profit. You get a higher profit from trading the higher quality production plans. It's a fun industry.