You can construct your own automatron from scratch or modify Ada. There seems to be o limit on how many robots you can create, as long as you have the resources. The default model is the trusty protectron but as you collect more mods or depending on the level of your perks like Science, Robotics Expert, Armorer, and Blacksmith. Access to better mods require higher level of Science and Armorer primarily so if you have these leveled up during the core game, then you'll be able to build heavily armed and better equipped automatrons early on.
You can mix-and-match various automatron parts and customize your own build. However, some parts are exclusive only to a particular model. For example, armor options for sentry parts may not fit assaultron parts. Weapons however, doesn't seem to have this restriction since you can fit any weapon to any model. So if you want to build a hovering sentry bot armed with melee weapons and launchers, you can do so. Let your imagination run wild.
Base Automatron Components:
Unlocking Parts and Mods
The available mods for your robots will depend on specific perks. Having a Science and Armorer perk rank of 4 will give you access to most of the mods. Additionally, you can unlock new mods by looting special parts or mods from defeated robots. You'll get a notification in the top right of the screen confirming if you have indeed acquired a mod that's not yet available in your Robot Workbench. If you receive another copy of that mod, you'll be able to attach it to your automatrons for free without spending components crafting one. Defeating the mechanist and taking complete control of the robotic facility he's running will unlock more mod options for your robot workbenches.