Macbook foto library vs whatsapp

Hi Guys, first time here:-)

Question:
I have been using WhatsApp on my iPhone for years. Recently purchased a MacBook.
Since I also use iCloud, it makes sense that the photo library on the MacBook is also in sync with the one on my phone.
Now I also see a WhatsApp folder with photos in it on my macbook.
However, what I can't figure out right away:

The photo library on my MacBook contains all the photos I take/recently deleted etc. on my mobile phone = clear.

The WhatsApp folder on the mac does not contain all the photos from WhatsApp? Some pictures can clearly be found in a chat on my phone, if I compare by date and time on the macbook in the relevant folder, I can also see them in a relevant chat. 
But this is not applicable to all photos?
It also seems pretty clear to me that this whatsapp folder only contains sent photos?
And is it possible that some of the photos in this whatsapp folder are not in my default photo library?
And should all photos in the WhatsApp folder be related to a chat? Or can they also end up here differently?

Anyone have an idea of ​​the difference and logic what is used here?

Thanks,
J