Pretty much everything with strict resource requirements or other reasons for aggressive resource use optimization: embedded, automotive, industrial control sytems, games, operating systems, audio and other (soft) realtime media applications, hft, tools and frameworks for all these things, etc
But you'll note that most of these things are very sensitive to quality of work. That is not something that people find easily on freelancing platforms like upwork and so people seeking contract help for these things don't post their work their.
Instead, buyers generally reach out through established connections ("word of mouth", past coworkers, approved vendor lists) or fall back to hunting through Google and LinkedIn for firms/individuals that look impressive, hoping that the impression holds up during a requirements gathering, bidding, and estimation process.
Given where it sounds like you are in your career, your best bet might be to find work inside a contracting firm that can feed you gigs (they'll do sales, they may do project management, they may staff a team alonside you, they'll take a large cut). If you make good impressions on the clients and colleagues you work with there, they will become the personal connections that feed you independent work some years down the road.
Without taking the time for that, it can pretty had to break in, and much of the work you may be able to find will be miserable table scraps for poor clients who barely know what they're doing themselves.
But you'll note that most of these things are very sensitive to quality of work. That is not something that people find easily on freelancing platforms like upwork and so people seeking contract help for these things don't post their work their.
Instead, buyers generally reach out through established connections ("word of mouth", past coworkers, approved vendor lists) or fall back to hunting through Google and LinkedIn for firms/individuals that look impressive, hoping that the impression holds up during a requirements gathering, bidding, and estimation process.
Given where it sounds like you are in your career, your best bet might be to find work inside a contracting firm that can feed you gigs (they'll do sales, they may do project management, they may staff a team alonside you, they'll take a large cut). If you make good impressions on the clients and colleagues you work with there, they will become the personal connections that feed you independent work some years down the road.
Without taking the time for that, it can pretty had to break in, and much of the work you may be able to find will be miserable table scraps for poor clients who barely know what they're doing themselves.