Call API in and out in Dynamics 365

Dear Friends,
I have a case:
1/ The ABC have a legacy Retail application (on-premise).
2/ The CEO want to purchase Dynamics 365 and integrate with Retail application
Question:
1/ I want to write the code from Dynamics 365 call API of Retail application. Could you share the code?.
2/ I want to write the code from Retail application call API of Dynamics 365. Could you share the code?.
Thank you.