checkout is a local command

git-checkout - Checkout a branch or paths to the working tree

Git can be used without an Internet connection, your understanding is correct

git checkout -f I tested it and the result is like this

rails@1hao:~/tmp/tmp$ git branch 
* hello
  master
rails@1hao:~/tmp/tmp$ git checkout -f 
rails@1hao:~/tmp/tmp$ git branch 
* hello
  master

In addition, the git-checkout man manual says this

 -f, --force
           When switching branches, proceed even if the index or the working tree differs from HEAD. This is used to throw away local changes.

Changed the file, added a file and tested it

rails@1hao:~/tmp/tmp$ git status
# On branch hello
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#   new file:   b
#
# Changes not staged for commit:
#   (use "git add <file>..." to update what will be committed)
#   (use "git checkout -- <file>..." to discard changes in working directory)
#
#   modified:   a
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   c
rails@1hao:~/tmp/tmp$ git checkout -f 
rails@1hao:~/tmp/tmp$ git status 
# On branch hello
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#   c
nothing added to commit but untracked files present (use "git add" to track)

The files that were changed and added to the cache are gone