Understanding git submodule pulling and pushing

Question

I have read the following articles:

1. Git Tools - Submodules
2. git-submodule
3. Git Submodules: Adding, Using, Removing, Updating
4. easy-way-pull-latest-of-all-submodules

But I still do not 100% understand submodule handling. My questions are:

1. What does git submodule update do? Is it the same as doing git submodule foreach git pull?
2. What is the difference to git submodule update --force?
3. What is the difference to git submodule sync? And when do I have to call update and when do I have to call sync?

4. This question is a bit longer. I had to trigger a git submodule update --init —recursive. But why did no git submodule update --force or git submodule sync help? Please regard the following history of commands and outputs. I am sorry for the terrible formating. But I am somehow not able to insert this as code.

           - cd $gitproject
           - cd submodule_a
           - git status
               - ad96e1a11:og-appconfig-android knoppik$ git status
               - On branch develop
               - Your branch and 'origin/develop' have diverged,
               - and have 1 and 5 different commits each, respectively.
               -   (use "git pull" to merge the remote branch into yours)
               - nothing to commit, working directory clean
           - cd ..
           - git pull
           - git submodule update --force
           - git submodule sync
           - cd submodule_a
           - git status
               - ad96e1a11:og-appconfig-android knoppik$ git status
               - On branch develop
               - Your branch and 'origin/develop' have diverged,
               - and have 1 and 5 different commits each, respectively.
               -   (use "git pull" to merge the remote branch into yours)
               - nothing to commit, working directory clean
           - cd ..
           - rm -rf submodule_*
           - git submodule update --init --recursive
               - Submodule path ’submodule_a’: checked out '8d4d756f2342bc373b6e86f0d0246f7b502ba796‘
           - git status
               - On branch develop
               - Your branch is up-to-date with 'origin/develop'.
               - nothing to commit, working directory clean
           - cd submodule_a
           - git status
               - HEAD detached at 8d4d756
               - nothing to commit, working directory clean
   

So why did git submodule update --force not work? Why did I have to delete the submodule folders and run an init once again?