Hi - we have developed an extensive library of custom REST endpoints for the project that I work on. The best way to get started is to use the Alfresco SDK kit for the version of the Content Server that you are using - that will set up the directory structure and Maven pom files that you will need in order to compile your endpoints. SDKs are available here: https://docs.hyland.com/r/Alfresco/Alfresco-Content-Services/23.4/Alfresco-Content-Services/Develop/...
If you have not ever written an endpoint before, the tutorial here should help you with how to do it.
https://docs.hyland.com/r/Alfresco/Alfresco-Process-Services/24.3/Alfresco-Process-Services/Develop/...
Depending on your requirements, it may be useful to set up an intermediate layer that can coordinate calls to Alfresco with other business rules, such as MuleSoft or Apigee.
