Using the right media players on raspberry pi can be pretty difficult. On every raspberry pi there is one preinstalled media player called VLC media player. If you click on a video file, this is what it will automatically open up to.
If you want to your raspberry pi to only be a media player, with no desktop or anything, there is an app and OS called Kodi that you can use that for it. To download the app look in pi-apps, then multimedia section. There you will find a app called Kodi. Once you install it, you will find inside of the sound and video section of your application menu.
When you open it, you will notice that your entire pi pretty much reboots itself. Once it loads you will be in kodi! To add music files go to the files section. You can also get music addons that will also do this for you. Kodi can also be used to watch TV, as a digital photo frame, gaming system, and more! Experiment around to see what you can do! If you want to install Kodi as a OS and write it to an SD card you can do it VIA raspberry pi imager. If you open imager, then go to media players, the libreELEC you will find that it is kodi.
Cairo-dock also comes with a media player as well. if you don’t have cairo-dock, click here to install it. If you right click on one of icons on your dock, then select cairo-dock, then add, then add applet, it will open up to this.

There is one called music player. Click the check mark and it will appear on your dock. Then when you want to open a video file, drag and drop it onto the icon.