Serving the CSS and JavaScript files of the Bootstrap theme locally
You need Bootstrap Library module.

Quote: 


  
  Download Boostrap files on /libraries/bootstrap (you`ll have to
  create "libraries" folder in Dr8 install directory).    
  Then go to
  example.com/admin/config/development/bootstrap_library 
  -configure on which pages you want to load the libraries (by default everything but admin/*).    
  Configure on which themes you want to load the libraries.
  Drink some champagne coffee.