Learn Danish: Har hun et krævende arbejde? - Does she have a demanding job?

This is exactly what the zero-agreement rule is all about! krævende is a present participle (a verb turned into an adjective, ending in -ende). Unlike normal adjectives, present participles never change their ending to match the gender or number of the noun. So it stays krævende and never takes a '-t' or '-e' ending.

How is the word krævende formed from a verb?

It comes from the verb at kræve, which means 'to demand' or 'to require'. To form a present participle in Danish, you generally take the infinitive form of the verb and add -nde, creating krævende (demanding).

Could I use the word job instead of arbejde in this sentence?

Yes, you absolutely can. et job and et arbejde are often used interchangeably when talking about a specific position of employment. While arbejde can also refer to 'work' in a general, uncountable way, adding the neuter article et makes it mean a specific job here.