In the respective factories you can build kbot (builder), vehicle (dozer) and air builders
Builders (construction robots) can only build missile (anti-vehicle) defenses
Dozers may only build laser (anti-kbot) defenses. They cost more, but move and build faster than builders. They may also run over lighter bots
Air builders cost more and build a bit slower than the others, but have unmatched mobility and can build both laser and missile defenses
Outposts and the commander can also build both laser and missile defenses
No standard (Tier 1) builder can assist (speed up unit construction of) factories, only their advanced versions
All builders can morph to their T4+ (advanced) counterparts
FARCs - Fast Assist and Repair Construction units
FARCS move and repair really fast, but are slow reclaimers (harvest resources from trees, rocks and wreckages in the battlefield) compared to the builders and outposts
All FARCs can assist factories, in fact they're the best early option to speed up combat unit production
FARCs may also build cheap dragon eyes (remote spy cameras)
Basic builders and FARCs can morph into Tier 2 versions (requires a Tier 2 tech center)