Go here https://github.com/Stromberg90/football-manager-tools and download the latest blender addon from the releases page, and follow the instructions.
Using the Football Manager 2021 Resource Archiver which comes with the game on steam under tools, extract simatchviewer and simatchviewer-pc, these are the models and textures that comes with the game.
If you have not installed blender this is a good as time as any, https://www.blender.org
Open blender, then go to Edit->Preferences->Add-ons->Install...
and select the zip you downloaded from github, after that enable the addon
then set the Extracted textures path to your extracted simatchviewer-pc folder, so that on importing a model it will assign the textures automatically.
Lets have a look at one of the changing rooms that's in the game, just as a example, open the importer like this.
Then we'll go to where you extracted simatchviewer and into \environments\changing_room\, there we'll find changing_room_large.sia which we'll have a look at.
If everything has gone well it will import the mesh, textures and setup some materials, which you'll see if you switch the viewport mode over to Material Preview or Render Preview mode in blender.
Hope this piqued your interest , and thanks for reading
The addon also supports exporting, which I encourage you to try out, and it might be the topic of another article.
If you have any issues contact me directly, or add a issue on github.