A robot is a chain of tolerances. Every joint interface, every arm segment and every end-effector mount adds its error to the chain, so the parts that matter most are the ones nobody sees.
Joint housings and covers, arm segments and links, gearbox and motor interfaces, end-effector mounts, gripper parts and base plates.
Aluminium for the balance of stiffness and low mass, titanium where strength-to-weight matters most, stainless where corrosion or wear resistance dominates, and machined composite components where stiffness per kilogram is the priority.
Bearing bores, gearbox mounting faces and joint interfaces decide repeatability. Machining them in one set-up where possible, and verifying roundness, flatness and position on a Zeiss CMM, keeps the tolerance chain short — which is the only way a robot stays accurate at the tip.