The Medical Tech (Medtech) skill is defined on p. 46 of the Cyberpunk 2020 book:
This is the skill used to perform major surgery and medical repairs.
For more descriptions of this skill in action, see Trauma Team, p. 116.
On page 116 of the core book, it elaborates:
To make a successful medical skill check, you must roll a value [...]
Medical skill checks are made with two skills, First Aid or Medical
And on p. 117, it further explains:
A character with Medical Tech skills makes a check as if using the
First Aid skill, however, with Medical Tech, the patient will recover
at the rate of 1 point per day. [...] Using Medical Tech skills supersedes the use of First Aid skills;
So yes, Medical Tech is used instead of First Aid, which is how I've always run my games (including several Trauma Team games) and any game of CP2020 I've played has done the same.