Questions about % of STATIC_URL="https://%s/%s/"%(AWS_S3_CUSTOM_DOMAIN, AWS_LOCATION)

S3 is one of Python's string expansion notation, regardless.

https://docs.python.org/ja/3.7/library/stdtypes.html#printf-style-string-formatting

If format %values (where format is a string), the %translation specification in format is replaced by zero or more elements in values.

How to embed variables in a string with Python
This blog might be easier to understand

I think the following writing method is easier.

STATIC_URL=f "https://{AWS_S3_CUSTOM_DOMAIN}/{AWS_LOCATION}/"