Good backend project to build in C++? (Any senior dev please help)

So-called "competitive" programming is almost always of no practical use when it comes to normal programming. Treat it as a hobby, rather than something that will teach you anything (except bad habits and bad code). Related to that, the term DSA almost everywhere else means something completely different.

It gets candidates' foots in doors. People need to stop looking down their noses at this sort of thing. Sure, writing awful code is a hazard of those sites, but it's not a guaranteed side-effect either. I've written 98th percentile code in time and memory that's perfectly acceptable in a professional environment. The micro-optimizations and garbage macros don't count for much (shocker!). It's a perfectly reasonable way to expose yourself to many good algorithms, and to practice things like dynamic programming.

I would recommend finding one or two open source projects, and contributing to those. Employers love to see contributions to open source. If you can get your code accepted in open source projects, you're showing that you do indeed know how to code, and other people in a group setting agree with that sentiment.

What happened to embedded systems? For example, mobile phones, medical devices, aerospace, automobiles, dishwashers, microwave ovens, and printers to name a few.

There's more out there than networking.

I’m good at DSA and competitive programming in C++

If you're going this route, did you write your competitive coding answers that looks like a professional or systems level coder wrote it, and not like the slop "answers" you see on many of these sites?

For example, those sites that shows other solutions -- a professional programmer would cringe, even if the solution gives the correct answer. One letter variable names, crazy #define macros, poor usage (if any) of the C++ library functions, etc.

A lot of those solutions you see can be done in a fraction of the number of lines of code if the coder actually knew C++ and the standard library, and not just hacking away.