We're glad you asked.
The Weblow API key is stored on Airtable's servers in secure storage (Global Config).
GlobalConfig storage is not shared across different users. It is individual for each installation within a base.
We are not storing your API key elsewhere and we are not passing it to any 3rd parties. It's 100% in Airtable.
This is a safe and recommended approach for storing API keys within Airtable.