Learn Hardcore Vietnamese: Họ không những vệ sinh mà còn khử trùng.

Breakdown of Họ không những vệ sinh mà còn khử trùng.

What does không những ... mà còn ... mean?

It is the Vietnamese pattern for not only ... but also ....

So in Họ không những vệ sinh mà còn khử trùng, the speaker is saying that they do more than just clean; they also disinfect.

The structure is:

This pattern is very common in both spoken and written Vietnamese.

Why is mà used here?

In this sentence, mà is part of the fixed pattern mà còn, which works with không những.

So you should learn the whole pair together:

It links the first action to an additional, often stronger or more impressive, second action.

You usually would not translate mà by itself here. It helps build the not only ... but also ... meaning.

What does vệ sinh mean here? Is it a noun or a verb?

Here, vệ sinh is being used as a verb, meaning to clean or to sanitize / do cleaning depending on context.

Vietnamese words can often function in more than one way depending on the sentence.
For example:

vệ sinh as a verb: Họ vệ sinh phòng học = They clean the classroom.
vệ sinh as a noun-like word: giữ vệ sinh = maintain hygiene / cleanliness

In your sentence, it clearly acts as an action: they clean.