I have worked with various different integration methodologies. Using APIs was definitely one of them. To be more precise, I am sharing some of the integrations that I worked with. 

API development for Internal Use: By leveraging the Python Django/Framework I created our own APIs for internal services communication and automation. 
https://www.django-rest-framework.org/
https://www.django-rest-framework.org/tutorial/quickstart/
https://docs.djangoproject.com/en/3.1/ref/

Integration with Visa: At my previous job with Accuralify I developed the Visa Credit Card integrations using their various APIs in order to create a Virtual Card, make payments and virtual transactions. Here are the links to some of the Visa APIs that I worked on. 
https://developer.visa.com/capabilities/vpa 
https://developer.visa.com/capabilities/vba
https://developer.visa.com/apibrowser

Integration with Silicon Valley Bank: Developed the integration with Silicon Valley Bank to comply with the request of clients working with Silicon Valley Bank. 
https://www.svb.com/developers

Integration with Oracle Netsuite: Worked on integration with Oracle Netsuite. Oracle Netsuite uses SOAP WSDL as their supported integration method. 
https://www.netsuite.com/portal/developers/resources/suitetalk-documentation.shtml

Integration with SAP: To accomplish the integration with SAP I worked with Remote Procedure Calls and IDocs. To Achieve this I worked with open source libraries such as HiberSap, JBoss. 
http://hibersap.org/documentation/

Other Integrations:
- Microsoft Active Directory. 
- Logo
- Used Google Maps api for a school project.