What is the best way to log Group SMS inside Salesforce?
is it like a task is created for all the contacts inside group sms?

Example:
For 1:1 SMS
Question:
What happens when an inbound message from an unknown number is received?(Is it like we just create the task without being tied to any record, contact, lead or account?

Answer:
We try to find a match in salesforce based on the Contact details(phone number). If there is a single match, we match the contact with that record, and we log the task. If there are 0 matches or multiple matches, we don't log the task.

Note: I am talking about Group SMS not Bulk SMS.