der Schlosser
metalworker
A tradesperson who makes, works on, and installs metal parts for structures, machines, or buildings.
Der Schlosser fertigte ein neues Geländer für die Treppe an.
The metalworker made a new railing for the stairs.

metalworker / locksmith
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metalworker
A tradesperson who makes, works on, and installs metal parts for structures, machines, or buildings.
Der Schlosser fertigte ein neues Geländer für die Treppe an.
The metalworker made a new railing for the stairs.

The word 'Schlosser' is derived from 'das Schloss'. 'Das Schloss' can mean both a door lock and a large building (palace/castle), which explains the two main activities of the profession: working on locks and doing metalwork for buildings.
Ein Schlosser muss präzise mit Metall umgehen können.
A metalworker has to be able to work precisely with metal.
You can tell which meaning is intended from the context. If it is about keys, doors, or locking yourself out, it means the tradesperson who works with locks. If it is about metal structures, railings, or gates, it means the metalworker.
Ich habe mich ausgesperrt und muss den Schlosser rufen.
I locked myself out and have to call the locksmith.
When stating a person's profession, 'Schlosser' is often used without an article. This is typical for job titles in German.
Mein Nachbar ist von Beruf Schlosser und arbeitet in einer großen Fabrik.
My neighbor is a metalworker by profession and works in a large factory.
mechanic
'der Mechaniker' and 'der Schlosser' both refer to tradespeople who can work with technology, metal, or machines. 'Mechaniker' is more suitable when someone maintains and repairs machines, devices, or cars. In everyday language, this often makes people think of an auto repair shop. 'Schlosser' is more suitable when someone makes, works on, or installs metal parts; sometimes it also means someone who opens or repairs locks. The words can be confused because both jobs involve practical work and may be encountered in workshops or around machines. As a simple guide: for engines and repairs, you are more likely to say 'Mechaniker'; for metal structures or locks, 'Schlosser' is more likely.

Der Schlosser montiert heute ein neues Metallgeländer am Eingang.
The metalworker is installing a new metal railing at the entrance today.
Nach der Arbeit bringen wir das Auto zum Mechaniker, weil der Motor komisch klingt.
After work, we are taking the car to the mechanic because the engine sounds strange.