Let’s start simple. I’ll get to how the various shared universes are connected to each other a little later, after establishing a few of them.
In the third season of the original Transformers, Marissa Faireborn is the daughter of Flint and Lady Jaye from GI Joe, and in one episode, Cobra Commander himself appears.
But additionally, the character Hector Ramirez appeared in Transformers, GI Joe, Jem, and Inhumanoids.
Additionally, in C.O.P.S. (not to be confused with COPS), Checkpoint is the son of Beach Head from GI Joe.
And in the Japanese Transformers series Headmasters (a split timeline from the US cartoon), the Transformers met the Battle Beasts (or Beastformers as they’re called in Japan).
Obviously, this universe also includes all the other G1 Transformers cartoons, Beast Wars and Beast Machines, and GI Joe Extreme.