Azure DevOps : Pull Request across the Repositories?
You can do such operation in Azure Devops. BUT, the prerequisite is there must exist Fork relationship between current(Repos-A) and target(Repos-B) repos.
In another word, one repos must be forked from another repos if you want to create pull request across repos.

At this time, you will has option to choose another repos/project while you creating the Pull request.

As normal, we does not very recommend such operation. In this doc, we described some scenario:
For a very small team (2-5 developers), we recommend working in a
single repo. Everyone should work in topic branches, and master should
be protected with branch policies. As your team grows larger, you may
find yourself outgrowing this arrangement and prefer to switch to a
forking workflow.
If your repository has a large number of casual or infrequent
committers (similar to an open source project), we recommend the
forking workflow. Typically only core contributors to your project
have direct commit rights into your repository. You should ask
collaborators from outside this core set of people to work from a fork
of the repository. This will isolate their changes from yours until
you've had a chance to vet the work.