You can link a work item in Azure Boards to a commit in Azure Repos by specifying the work item ID after a pound sign. Eg, to update work item #3, you need to mention #3 in the commit message. (The pound sign – # is required, it won’t parse bare numbers and try to link work items.)
For example:
git commit -m"Update bug #3"
Produces a bidirectional link between the commit and the work item. You can see this in the commit details, in the work item tab:

And you can see this in the work item details, in the “Development” section:
