how to become a Linux expert?

it looks like you need to learn a lot. There is no documented way and also you can use a lot of different resources like courses/tutorials/books. But first of all you need to practice a lot.
Also you can check the bottom of the page and you will see a few similar threads.

Sign up for one of the certification courses from a major player like RH, SUSE, etc. This is the easy way. It also generally costs a bit. Some certifications run $400-$500 just for the test alone. See RH examples here https://www.ubuntupit.com/best-linux...s-and-courses/

Alternatively you can go with one of the education specific options. Check your local institutions of higher learning, or pages like this https://hackr.io/blog/best-linux-certifications.

Slowest, but least expensive is to study on your own using the free courses and tools online. Options are easy to find, a quick search turned up https://itsfoss.com/free-linux-training-courses/ and https://training.linuxfoundation.org.../free-courses/ that look interesting. Of course, once you study enough that you think you will pass a test you still need to schedule and pay for a certification test. Also most certifications expire, and you need to study and retest every few years: especially in the security related fields.

If you have an enlightened employer they may pay for either the course or the test, or possibly even both. Mine paid for the AIX System Administration certification test, but I had to seek out and study all of the AIX course material on my own, or pony up for the classes out of pocket. That was, however, back in 2000 and that employer did not survive as a company to today. They were "aquired" by a corporation, and then sold off to another long since. All I can do there is wish you the best of luck.

please do not change your original post (including title) to a completely different topic.

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OP is a spammer.

They've just edited their post from a question about becoming a Linux expert and finding course syllabuses to "What are the best reasons to use Linux?"

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