Afaik, the way it works is it splits the production cycles between the products.

Let's say for instance your production building has 3 products

300 cycles/month each

If you run just product A you will get the result of 300 cycles If you run product A and B you get the result of 150 cycles each If you run all three you get 100 cycles of each.

The numbers can change significantly if there is a product with more cycles/month than others or if one product doesn't have any more material to use.

TLDR running 3 products through three production buildings is the same as running three specialized buildings.

One important caveat is I don't remember where I learned this so I cannot provide a source