How to keep a branch synchronized/updated with master?

yes just do

git checkout master
git pull
git checkout mobiledevicesupport
git merge master

to keep mobiledevicesupport in sync with master

then when you’re ready to put mobiledevicesupport into master, first merge in master like above, then …

git checkout master
git merge mobiledevicesupport
git push origin master

and thats it.

the assumption here is that mobilexxx is a topic branch with work that isn’t ready to go into your main branch yet. So only merge into master when mobiledevicesupport is in a good place