However, automating manufacturing processes has also reduced the number of jobs [which / in which] workers can earn a decent living.
If you put: 'jobs [which] workers can earn a decent living.' we read 'which' as a relative pronoun for 'jobs' and an interrogative determiner for 'workers'. This is a clash, as it can't be both at the same time, and renders the clause ungrammatical.
In English it is common to use 'in' when talking about jobs:
Are you in work at the mo? Are you in employment? Which job are you in? In what profession are you?
'in which (jobs) workers can earn a decent living.' I tihink this is an adjective (although it looks a lot like an adverbial to me!) describing 'jobs', because you could replace it with 'available', which cannot be an adverb. However ,I wouldn't like to rule out the possibility that it is the adverbial that answers the question: Reduced the number of jobs where? Answer: in which workers can earn a decent living.
It can't really be both at the same time, can it?
*by which workers can earn a decent living. Don't like this for the reasons stated above: in a job.
*through which workers can earn a decent living. Ditto.
