I just got Ada as part of the Mechanist DLC for Fallout 4. So far, all I can seem to do for upgrades is some basic assaultron and protectron components. How do I give her more advanced parts like sentry bot or novatron?
Robot parts can be looted off of other robots you destroy, or they can be crafted just like weapon mods or armor mods through finding junk and creating it. Crafting the parts requires the Robot Workbench, which you can build after the Mechanical Menace quest.
Its similar to how if you have junk in your workbench at a settlement. When you go to modify the robots at the robot workbench, any mods you can build will be available for you.
A complete list of all the parts you can find/make can be found here at the Wiki (its rather long to list here).
Robotic parts can be made as you defeat and find more advanced bots. Encountering a robot with a certain part, defeating it, and looting said part unlocks the blueprint/schematic for that part.
If you have completed the quest where Ava gives you the robotics workbench then you can carry on roaming around freely if you don't have enough scrap to build you don't have the perks for it as you will find some robot groups either attacking people or roaming freely if you kill them successfully then you can loot them for a chance at a certain robotic part which then you can craft when you want and equip to any robot
There is actually another way I always go. Just finish the all quest in the automatron DLC (there are like three overall) to get ALL available parts. This usually takes like 3-5 hours and is much faster than collecting the parts on their own.