I thought boto3 is dedicated for only AWS services such as S3, EC2, and more and then have found that Linode S3 document is guiding to use boto3 as well.
How can Linode S3 API can be interacted using boto3?
I thought boto3 is only for AWS service as mentioned as "The AWS SDK for Python (Boto3) provides a Python API for AWS infrastructure services." in AWS SDK for Python (Boto3) Documentation.
Can Boto3 connect to S3 object storages that is from other Cloud provider, not only Linode, if the service is following appropriate API structures or so?
You do this by overriding the S3 endpoint URL in the boto3 configuration, to point to the Linode endpoint instead of the AWS endpoint, as it clearly shows in the Linode documentation you linked in your question.
Since Linode has created an "S3 compatible" API, which basically copies the AWS S3 API, it is possible to use tools designed for AWS S3 with Linode. There are several other services, like Backblaze B2 and Cloudflare R2 that are also S3 API compatible.