First, you're going to open your Counter Strike: Global Offensive game directory. You can do this by either opening your Steam games directory and naviagating to:

\steamapps\common\Counter-Strike Global Offensive\csgo\panorama\videos

OR
You can right click CS:GO > Properties > Local Files > Browse Local Files

Then navigate to

csgo\panorama\videos

In this folder is the assets used in the main menu of the panorama U.I. Notice the file names and the several versions of each file.
https://gtm.steamproxy.vip/ugc/1048722385372561187/8846D9C2621CD48DE143EF363EFC5EBD606C7A09/?imw=256&&ima=fit&impolicy=Letterbox&imcolor=%23000000&letterbox=false

Navigate to this website[ymp4.download], this one works for me

WARNING: I am not affiliated with this website, any navigation you do here and any content on this website is not my responsibility

There are ads, so just be smart and don't download .exe's

Paste in your YouTube video link

If you have a .mp4 that you would like to upload instead, use this website here[convertio.co]

IMPORTANT: Make sure to convert as "VP8". Otherwise it will not work.

On the first converter link, look for "More video" and click the "WebM VP8" download button.
If a popup comes up, close it and click download again. Do not download anything that is not a WebM.

Place your newly downloaded video in the directory you created. For main menu videos, replace any of the supported maps' files.

Maps availble to change as of 5/27/2020: Anubis, Blacksite, Chlorine, Cobblestone, Nuke, Phoenix Facility (called Sirocco Night in files), Vertigo

Delete the regularly named file (Not the xxxx540.webm or xxxxx720.webm)
Note: If you're using 720p or 540p resolutions, you need to change the respective file as well.

Rename the newly downloaded WebM as the file that you just deleted.

Go to the settings page and change the "Main Menu background scenery" to the map you changed

You're done!

If you want to change the intro, the matchmaking world, go button, inventory acknowlege and more; do the same process but replace the file names

If you need some help, please feel free to comment!