Most of the time, what I notice in freshers is lack of basic understanding. For instance, they can solve a complex problem of Dynamic Programming (I guess by practising a lot on Geeksforgeeks etc.) but they can't really tell how a simple HTTP request is handled, or what is a simple server client architecture.
I can't say for everyone, but at least I like to gauge the basic understanding of the candidate instead of testing them in some complex programming problem. For instance, I would limit myself to a simple BFS, but that BFS will essentially be wrapped under a problem, which u would only be able to solve, if u understood BFS.
IMO, practising at these sites is essential for learning about the problem, but your primary focus should be to actually understand it.
Learn the basics of networking, operating system, etc.
As a fresher, it is more likely that you will be asked a problem which gauges your understanding than some complex algorithmic problem.