If you're interested in the personal repositories of user X, you can go to https://bitbucket.yourdomain.com/users/X to get a list of their personal repositories you can access. I
If you're interested in the private repositories of all users, you'll have to use a REST endpoint. There's no UI that provides this. See https://developer.atlassian.com/static/rest/bitbucket-server/4.0.2/bitbucket-rest.html#idp727152 for details.
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