Yes

are these terrans

They are basically making their choice from a large collection of prefabs. First, they make sure they are even able to build it (i.e. they are the right race, have access to the right resources and components, etc). Next, they weigh all the available prefabs to determine which would be the "best" for them at this time. The "best" is determined using a statistical analysis of the grids that faction already controls.

In summary, they usually build 50% workers and then the most expensive grid they can actually build with what they have available. They try to have about 1/4 static grids but that's not always the case for all factions.

im interested in how the vehicles they make are decided? do they have a set of blueprints? or do they just have to follow a set of rules?

It was really just added to spice things up but glad you like the idea! It's probably also worth noting that any faction could become an AI controlled faction that acts however you want. For instance, if you had a faction called ABC, you could turn it into a similar "grounded" faction using the following chat command:

/sc establish ABC -grounded

that sounds amazing for a rover wars server. Suscribed.

Yes, this faction has the "grounded" flag which prevents them from building flying units. It's a little hit or miss but that's how it works. To compensate, it does gain access to Uranium and Platinum after building its first large refinery due to its "nuclear" flag.

do they only use rovers?

This faction now accepts humans by default without needing a chat command.

Discovered by default and gained the Miner tag