In your opinion, what is the best way to do research to find out the technologies that are in demand, specifically in the context of short term projects/freelance marketplaces (let's say in the data engineering domain for example). I don't think a lot of those marketplaces publish their open project requirements to non-members. (background: I have some downtime that I would like to use for learning)

StackOverflow survey results is a good start. Apart from that, you can follow popular job boards to find a pattern of what's most common in requirements and then narrow down what you can focus to learn.

Check out Hacker News' Who's Hiring post. It should give you a pretty good idea about skills in demand, from a global perspective (heavy US bias, but that's the hub of most new global tech)

Also, JS, PHP, Ruby, Python, and Go/Rust seem to be evergreen. 😄 (you can include Java and Swift/Obj-C too if you count mobile apps).

Thanks @flat-morning-91037 Didn't know about SO surveys, will check out. Remote job boards are good, except i don't know if there is a considerable difference between full time remote positions vs short term contract projects like the ones those gig platforms focus on