Where are the 'Contributions - Receipt (on-line) templates stored?
Do you mean the code that uses the template?  If so, look at CRM/Contribute/BAO/ContributionPage.php and look for 'contribution_online_receipt' which is the template name for 'Contributions - Receipt (on-line)'.
Unless you want to pass different data to the template, you should just need to edit the template at Administer &gt; Communications &gt; Message Templates &gt; System Workflow Messages
If you change the templates you might find this extension useful when you need to reconcile future core changes with your changes: https://lab.civicrm.org/extensions/msgtplhelper
And if you're making your templates look better, try this: https://lab.civicrm.org/extensions/prettyworkflowmessages