Learn Spanish (Spain): Trabajo mucho en la ciudad.

In Spanish, the ending of the verb tells us who is doing the action. The verb here is trabajar (to work). Because it is a regular verb ending in -ar, we drop the -ar and add -o to say "I do it", making it trabajo (I work). You can add yo (yo trabajo) for emphasis, but it is usually left out!

I learned before that el trabajo means "the job". Why is it used as an action here?

Trabajo can be both a noun and a verb! As a noun, el trabajo means "the job" or "the work". As a verb, trabajo means "I work" (from the verb trabajar). You can tell it is a verb here because there is no el before it, and it is describing the action you are doing.

Does mucho change to mucha because la ciudad is feminine?

No, because mucho (a lot) is describing how much you work (trabajo), not the city itself. When a word like mucho describes an action (a verb), it never changes its ending. It only changes to mucha, muchos, or muchas when it describes a noun (like in muchas ciudades, meaning "many cities").

Why is it la ciudad? I thought feminine words usually ended in -a.