Add new Administration menu item in WordPress

although I have some skills in WP development, I do not know how to create new (page and post-like) Administration menu. I can see them in purchased themes only.

I want to create the new item here:

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On click on such item, I would like to see page-like table, possibility to add new items (possibly save such items in the new db table) and use this data elsewhere. I cannot past any code because I don’t even know how to Google this. Can somebody tell me where start?