Licensed engineers can through professional qualification and development gain approvals from an approved maintenance organization (145) to certify work carried out to an aircraft, its components and whole aircraft, subject to the approved capabilities of the approved maintenance organization.
Whether a licensed engineer turns spanners depends on trade, fixed wing, rotary wing and many other factors of the industry.
Technician or mechanics generally do turn spanners and may gain certification privileges from their maintenance organization. These however are usually confined to limited or simple tasks, normally associated with line work.
I have generalized...