The exact steps for adding an audiobook to iPhone 2020 SE

On contrary to what [I wrote half a year ago](https://apple.stackexchange.com/a/406162/46952) and what [someone wrote ten years ago](https://apple.stackexchange.com/a/11308/46952) this is again **not working**. I think I have passed all the prerequisites: 1. I am using _the same account_ on both my PC (iTunes) and my Apple iPhone 2020 SE. 2. Both devices are physically connected and authorised. 3. I am logged in to my account in iTunes for PC. 4. iTunes can see my device and library. The thing is that my library again is empty and that I can't add anything to it. Because: 1. I click _File > Add a Folder to Library..._, I select a folder full of .mp3 files, I click OK and... nothing happens. My library is empty. 2. I click _File > Devices > Sync "iPhone (name)"_, iTunes is quickly "something" (I see quickly changing steps from 1 to 4), but again no file is being transferred. Actually, I have done this about 60 times during past two hours. All for nothing. What am I missing or doing wrong? Can someone explain me, _step-by-step_ this (extremely terrifying, as it turns out) process of copying an audiobook / a folder full of .mp3 files from my PC to my iPhone? Exactly and step-by-step, please. What should I do (when to connect devices, when to start iPhone)? Where should I be (at which screen -- Library or iPhone)? What should I click and in which order? Thank you!