The Tech Rating of a unit is the highest Tech Rating of the components of the unit. The Tech Rating for components are listed along with the construction rules for the component, so most are in Tech Manual. SO p. 175 has an example for the AXM-1N of determining the Tech Rating of the AXM-1N as part of example maintenance on that unit.
SO p. 169 has an example maintenance with an Atlas and a Clint. Both are Tech Rating D. The Clint has a Quality Rating of B (and the Atlas a D). Quality Rating starts at D (Average) or depends on Faction/Era (see SO p. 167), but is likely to decrease over time. The Clint's Quality Rating is lower because it's "a battered veteran of the Succession Wars", not because it's a Clint. The Atlas' Quality Rating is still D because it's "fresh off the Defiance Industries production line".