so i just noticed during a campaign match against the coalition that their brain was using a tool that placed blocks behind it while it was digging? what tool is that and can a player use it? if so how?

The Constructor can be found in the tools section. It's visually identical to the concrete sprayer.

ok, yes i found that after i posted but all it seems to do is dig for me? is there a button i need to push b4 it will start building for me?

Yeah, check the pie menu. There should be an option like "order construction" or something. When it digs, it's storing material to use in construction.