I started looking for jobs that intrigue me technically, like for example: "eBPF developer" "Linux Agent Developer' "Linux Internals Developer" "Senior Software Engineer, Linux Kernel" "Senior Software Engineer, Filesystem" "Senior Software Engineer, Protocols"
to some of the above I got referred by friends who vouch me professionally and to some I applied online. Without exceptions, everyone declinded me for not having enough experience related.
I am wondering to myself, as a senior developer with so many projects on his belts in so many different areas of software development (that obviously don't fit a single CV page) why would no one consider me? of course that I'll have some learning curve in those jobs but who won't? It goes without saying but any backend/frontend/fullstack job accepts me without blinking.
What do you suggest me to do? how can a senior software engineer shift his area of expertise without going through being a junior again
Find something related act as a senior eng on.
That could be an internal project, an open source project, or a side project. Without more context, I'd suggest finding an open source project that interests you. If you can use it at your work, all the better. Spend some time improving that, learning the community, bug fixes, working on docs/workflow, etc. You know, senior engineer kinda things.
Then, next time you are applying, you can point to that project.