Creating Custom API endpoint for Sharepoint data

Hi,
I have a Canvas application named Skill Application to track a team members skill set, project details (completed/on-going) and some personal information. For this application I have made use of SharePoint as the data-source to store the details in the respective sharepoint lists (like Education has its own list: Qualification, etc).
Now what I wanted to do is expose the data source (in this case Sharepoint) as an API endpoint along with some authorization (to avoid access by anyone), so with this API url the designated person or it can be some other application (that is not developed in Power Platform) should be able to CRUD operations (retrieve the details, make updates, add new records, etc)

Employee_Projects is one of the SharePoint list ( there are around 7-8 SP list) which I want to be exposed as an API endpoint with some validation (to avoid unauthorized access) and how to add some logic (like if there is no record that is present in the given list then create a new one ; if the other app is trying to update an existing record  like particular attribute/column value) how to do that.

I have come across few methods which are registering the SharePoint application (that generate the Client secret and client id) and the other one is in Azure AD
Reference:
How to Call SharePoint REST API from Postman using OAuth2 Authorization Flow 
Accessing SharePoint Data using Postman (SharePoint REST API) 
[the above 2 links require to have appropriate permissions to proceed // id and secret key creation]

So alternatively I did found one documentation: To create custom API
Custom API doc 

But currently I am not sure this satisfies the requirement, so if anyone has an idea about this topic do let me know.

Regards,
Sidhant.