Can we connect to google cloud project, without using Client Library?
What is BigQuery API? Is it similar to bq-command line-tool?
If not, is there any way to convert bq-commandline-tool to API for usage in Python?
It is highly recommended to use the Google Cloud client libraries when available, but if you wish to call a Google Cloud Platform REST API from Python without a client library you can use an HTTP library such as Requests.
One thing client libraries handle for you is authentication. To authenticate to a Google API with Python, I highly recommend the google-auth Python library. Without an authentication library, you would have to implement the OAuth2 spec yourself, which is discouraged.