How can I transfer an audiobook to iOS18 without losing files

My situation: I have a few .aax files I would like to transfer to my iPhone with iOS18.3.

My mac has Sonoma 14.7.4 and Music 1.4.6.32.

Before, I used to import the .aax file in iTunes 12.8.3 on a very old macbook pro and could manually transfer it to an older iPhone with 15.8.3. And for that older iPhone, it still works.

But I would like it to work it with this new iPhone with iOS18.3 – which can’t be connected apparently to iTunes12.8.3 (I get stuck in “Downloading 1 item, about 0 seconds remaining”) – on my new macbook pro with Sonoma and Music/Books.

I already found that audiobooks no longer are imported in Music, but in Books. So I imported successfully the .aax file in Books. But how do I transfer it from my Books to Books on my iPhone with iOS18.3?

1. I tried to share the file from my macbook pro with Airdrop, only to find that the .aax file will end up in the Files folder, and can’t be reproduced nor saved to Books.
2. I looked up this Apple support page:

Sync audiobooks between your Mac and iPhone, iPad, or iPod touch - Apple Support

But I have a problem with syncing: I don’t want to sync EVERYTHING between my macbook pro and my iPhone. I just want selected files to sync.

So, when I tick the option “Sync audiobooks onto iPhone ”, with the intention to tick “Selected audiobooks”,

I get the alert that existing files will be removed. I don’t want to remove anything, I just want to add an .aax file!

I never understood nor liked the sync function for this, I am a manually managing apple user.

My mobile devices just should contain the (audio/book) files of my choice.

Thanks in advance.