Azure has very nice function to facilitate cross search through all repositories:
I have noticed that after I did search for a keyword which was in one of the branches it was not found. How can I cross search also through all the branches from all repositories?

When searching for code in Azure DevOps, it is possible to search fast because the default branch of each repository has been indexed.
You can extend which branches are indexed, and therefore will be included in the search by navigating to
Settings -> Repositories -> {{ choose repository }} -> Searchable branches, allowing you add a maximum of 5 more searchable branches per repository. It may take several hours your your newly added branches to become searchable however, as the indexing may take some time. See docs here.However, it appears you can still only choose one branch to search at a time. If this doesn't meet your needs, I would recommend searching for help on "how to search for code across all git branches", and use a method outside of what Azure DevOps offers. Here is an example stack overflow thread that may be closer to what you are looking for.