Sounds like the onboarding process needs to have a step in it that says "here's a link to a git tutorial, read this and get familiar with using git, as it's an integral tool that you will use every single day on the job". Bonus points for providing a sample repo that juniors can use to mess around with git, extra bonus points for including steps in the onboarding materials for the juniors to set up their own repos to play around with.