Currently, there is no feature to set global project permissions (aka workspace level permissions) in Bitbucket Cloud UI where you can apply permissions to all projects all at once.

Enhancement request:
It will be helpful to have the ability to set global project permissions in Bitbucket Cloud UI.

Workaround:
As a workaround, this can be done via REST API:

/workspaces/{workspace}/projects/{project_key}/permissions-config/users/{selected_user_id

API Reference: https://developer.atlassian.com/cloud/bitbucket/rest/api-group-projects/#api-workspaces-workspace-projects-project-key-permissions-config-users-selected-user-id-put