Hello
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For #1, enabling general_log and setting log_output flag to FILE will produce log output using the logs viewers in the Google Cloud Platform Console as indicated here. Additionally, if you wish to download these logs locally, you may refer to the documentation found here as these are the official instructions for downloading the logs into the JSON or CSV format. However, you can directly export the log entries to BigQuery using GCP console. You can do this by creating a sink and by specifying the destination being BigQuery as indicated here.
For #2, I believe specifying a schema for Bigquery would allow you to uniformly structure your logs between both products. You may feel free to explore setting up a schema for the logs and determine if they are applicable in your use case of structuring logs like mysql.slow_sql.
If you have any technical specific issues or inquiries related to this, you may find it suitable to post on Stackoverflow with the appropriate Google Cloud Platform tags. This action will ensure you would be getting the best possible support from the community for any of the issues you have encountered.
I hope this helps!