Hi @UQ - Jayesh Tikhe,
I will recommend you to register SharePoint App only to access SharePoint on premise. You can follow the document to register SharePoint App only.
https://learn.microsoft.com/en-us/sharepoint/dev/solution-guidance/security-apponly-azureacs
You can also follow the steps to perform rest api in postman
To learn more about rest api you could refer to the following document
https://www.codesharepoint.com/rest-api/0/all-methods
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