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The request signature (assuming version 4) is SHA256, the object eTag is MD5.

Yes, S3 uses the content-md5 header to verify data integrity for puts.

https://docs.aws.amazon.com/AmazonS3/latest/API/API_PutObjec...

For the ETag, yes and no that it's an MD5 of the object depending on encryption and multipart upload: https://docs.aws.amazon.com/AmazonS3/latest/API/RESTCommonRe...

For the object upload, AWSv4 request signature uses SHA256 on the payload/object, but I don't know if S3 also computes & compares the digest or just uses the x-amz-content-sha256 header value.

AWS support via Twitter said that the SHA256 digest of the payload is calculated and compared for S3 when using AWSv4 signature requests.