Recorded videos not shown in videos app

[lowrider2107]

My short search for this topic didn't return any results therefore I ask for your expertise here:

I have a number of videos stored on my phone adding up to 5 GB in total.
All except one I either recorded myself or received them via whatsapp.
Just one was transferred via USB from PC to the phone (800 MB).

Now these videos take up quite a lot of space so I wanted to go through them and delete some from my phone.
They are being uploaded to Onedrive anyways.

(1) When drilling down from the phone storage app (claiming 5 GB of videos) the video app opens and shows only the one file of 800 MB.
Why are my own recordings and whatsapp videos not treated as videos in the stock video app?
This is especially irritating as the storage app adds my own recorded videos size-wise to its video section...

(2) The files app lists the one transferred video in the "video" folder.
My recorded/received videos are listed in the pictures/camara roll folder or the pictures/whatsapp folder.

(3) In the pictures hub my own recordings and the whatsapp videos are shown along with pictures sorted by time.
No way to sort by type to get an overview of only the videos.

(4) Moli player behaves different again. It lists all my own recordings. Not the received ones and one transferred from PC.

This is all a terrible mix-up... Am I overlooking something here?
I just don't get the idea behind this, esp. (1) is strange...