The resource that you need to get access is in the other project.
roles/bigquery.dataViewer BigQuery Data Viewer
When applied to a table or view, this role provides permissions to:
Read data and metadata from the table or view.
This role cannot be applied to individual models or routines.
When applied to a dataset, this role provides permissions to:
Read the dataset's metadata and list tables in the dataset.
Read data and metadata from the dataset's tables.
When applied at the project or organization level, this role can also enumerate all datasets in the project.
Additional roles, however, are necessary to allow the running of jobs.

Reference: https://cloud.google.com/resource-manager/docs/access-control-proj The primitive role roles/viewer provides read access to all resources in the project. The permissions in this role are limited to Get and list access for all resources. As we have an out of the box role that exactly fits our requirement, we should use this.
Ref: https://cloud.google.com/resource-manager/docs/access-control-proj It is advisable to use the existing GCP provided roles over creating custom roles with similar permissions as this becomes a maintenance overhead. If GCP modifies how permissions are handled or adds/removes permissions, the default GCP provided roles are automatically updated by Google whereas if they were custom roles, the responsibility is with us and this adds to the operational overhead and needs to be avoided.

https://cloud.google.com/kuberun/docs/rollouts-rollbacks-traffic-migration

https://cloud.google.com/bigquery/external-data-sources
An external data source is a data source that you can query directly from BigQuery, even though the data is not stored in BigQuery storage.
BigQuery supports the following external data sources:
Amazon S3
Azure Storage
Cloud Bigtable
Cloud Spanner
Cloud SQL
Cloud Storage
Drive

Storage Admin (roles/storage.admin) Grants full control of buckets and objects.
When applied to an individual bucket, control applies only to the specified bucket and objects within the bucket.
firebase.projects.get
resourcemanager.projects.get
resourcemanager.projects.list
storage.buckets.*
storage.objects.*
https://cloud.google.com/storage/docs/access-control/iam-roles
This role grants full control of buckets and objects. When applied to an individual bucket, control applies only to the specified bucket and objects within the bucket.
Ref: https://cloud.google.com/iam/docs/understanding-roles#storage-roles