design requires bunch of computers and suitable software (electrical, mechanical, PCB design, several software development products, database..)
mechanical parts can be made using CNC
different machine will be needed for cleaning, painting etc.
then you have circuit boards manufacturing, testing, assembly, functional testing, high pot testing, data logging, communication, programming, etc.
then you have electrical assemblies and harnesses, cable assemblies, motors and sensors.
mechanical measurement will need hardness testing, CMM, X-ray to find defects and cracks,
then there is torque and force sensing along with bunch of other tools needed for assembly, testing, diagnostics, etc. (data loggers, data analyzers, oscilloscopes, programmers).
for only making one or two robots one can get away with a lot of time, patience, etc. many things can be done with manual tools but ... once you start thinking of volume production and testing, there will be a bunch of custom machines required or you will never get price point where they are marketable. so you need a factory and team or people.
next to you need to do a burn in test to estimate longevity and identify high wear components.
and when all is done you still need to think of packaging, documentation, IDs, tracking, support.
so you go through all of that and think you got a product? not so fast.
there is further testing to get approvals and not in one but at least several countries. that mean passing robotics safety standards. machine safety standards, meeting electrical standards (US/CSA/CE...), must also have some IO or preferably support at least couple of fieldbus systems.
so really you also need an investor, factory and small army of people.