Learning methods for secondary development of various systems

I have been familiar with php for a long time on the secondary development of various systems. It is no problem to write a simple website. I would like to ask where the popular CMS management system should start from ,. But I don't know how to do it. For example, discuz. PHPcmsdedecms and so on. In the end, how can we learn and make sense without detours? After one-click installation, I don't know how to learn about secondary development of various systems.
I have been familiar with php for a long time. It is no problem to write a simple website.
I would like to ask where the popular CMS management system should start from ,. But I don't know how to do it.
For example, discuz. PHPcms dededecms and so on. In the end, how can we learn and make sense without detours?
After one-click installation, I don't know what to do.
I hope you will provide some learning ideas. thank you very much.

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Learn how to use it first. Let's take a look at the principles. Go to the document. Then you can use the project to learn.
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Read the official documentation ....
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Find a job. A project is ready.