I think we are all pretty clear on the differences between the different form-factors (like Micro, SFF, and Tower), 

but how are the 30x0 , 50x0, and 70x0 series different from each other?

The third number in 3070, 5070, and 7070 is the "Release version" sorta-like a year-model (higher is newer).

This article should basically carry-over to other release-years. I thought it was good and might help yall shopping new or refurbished Optiplex models.

https://www.hardware-corner.net/guides/difference-optiplex-3070-5070-7070/