Looking to override AffiliateWP templates from a custom plugin instead of your child theme? This will be useful especially when using Oxygen since it disables the theme completely.
AffiliateWP provides a filter for that.
This members-only tutorial provides the steps on how to add a directory inside a custom functionality plugin to the list of paths that AffiliateWP checks for overriding its templates.
Note: It is better/easier and recommended to use actions and filter hooks where possible to customize plugins' output where possible. Template overriding should be the last resort.