I will try to explain to you what's the matter, the situation is this, my manager wants to use the API of another site for SharePoint on Premise 2019, so that those who find documents on another site with one click can call and write to SharePoint, the programmer of another site will write API but how to communicate with SharePoint I have to give documentation. I hope I have explained my problem. thank you very much.
There is plenty of documentation online which details SharePoint's client side object model:
More information can be found here: