There are a few options here.
You can use a Business Rule.
https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/data-platform-create-business-r...
This has some basic mathematical capabilities, and can sum fields together.
NOTE:
- This typically runs on the form itself, but you can/should set it to run at Entity scope (which will let it run on all forms, and on the server for updates/imports)
- You may need multiple rules, and need to plan correctly to ensure that the rules execute in the correct order
You can define column C and D as calculated fields, to do this automatically.
https://docs.microsoft.com/en-us/powerapps/maker/common-data-service/define-calculated-fields
NOTE:
- There is a limit of the number of calculated fields you can have
- Calculated fields are read-only, and derived when the field is read (data is not actually stored)
- Cannot trigger workflows/Flows/etc.
You can use Power Automate to update those fields, with the calculations you discussed. This would mean that you are using regular (non-calculated) fields/columns, which a Flow would set.
NOTE:
- Depending on the volume of data, and frequency of changes, then Flow runs may be a problem
- This is asynchronous, meaning you won't see the fields being calculated and would need to refresh the record after the Flow completed
