Taking content (mp4 and mp3) from usb drive and use it in a application
Various methods and it depends on how you want it done. You can:  

Run a CLI command finding .mp3 files through regex. E.g. find /media | grep *\.mp3:
This will automatically find all .mp3 files in the folder and subfolders.
You can run a CLI command inside your Python script e.g. with the subprocess module. Save all the available paths to a mp3 file in an array and process them further from here.
You can code a relative path in your program and mount your USB into your working directory's subfolder audio. mount -o remount,ro /dev/sda1 ~/path/to/your/project/audio:
All the audio files are then located in your audio folder which is part of your project. You can have dummy files in there and when you mount your USB into that folder the USB mp3 files are shown.
This approach requires you to manually mount before executing your Python program. It is also in my opinion the cleanest approach.

The manual mounting on the second approach might become a concern for you but for this there are also solution out there where you can define automatic mount points. However these aren't solved in Python.
I making a application, in this application you can watch movies and songs. 
For this i'm using angular for frontend and python/flask for backend.
Eventually this application will run on a raspberry pi 4, to store the mp4 and mp3 files i'm using a simple usb flash drive. For now i just want it to work on my laptop. So the program needs to know where to get the files from and render them in de html.
What is the best approach to load in the mp3 and mp4 files from that usb to display them in the application?
On first hand I just thought to give the path where the usb drive is located and then put that in de audio or video tag. Bu i'm not sure this is the right way.
Does anyone know what would be the best approach to do this?