have anyone work on one from the 90s and if so do you know the name of it?

am asking this to give the story the 90s real just right, if they didn't have them on the market during the 90s but just make there own from scratch I can work with that.

i know the single-board computer in the 70s as dev kits or training kits like the KIM-1 to make the PET system

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Without finding and/or tearing apart some of my old stuff, I’d think that stuff like the Timex Sinclair or possibly the Commodore 64 would likely qualify. It’s also possible that early laptops or portables might fit the qualification, as well--some of those did offer expansion slots and others did not. I think that my Compaq Portable III only had external expansion available, not internal (but don’t remember for certain).

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Atmel released their AVR family of microcontrollers in 96. The first one was the AT90S8515. I couldn't find many dates for other chips in that family, but they were probably early 2000s.
They were definitely around before then. I've worked with a Motorola microcontroller from the 80s but I don't remember the name.
Keep in mind that a microcontroller is idea differently then a regular computer though.