People often confuse "XML integration" and "API integration". They are different: XML is a data format, while an API is an interface systems use to exchange data.
XML
A structured format for storing/transporting data (e.g. a product list). Not a connection by itself.
API
An interface where you send requests and get responses in real time. See what is API integration.
Which in marketplaces?
Marketplaces usually offer both an XML catalog and an API; good integrations use both. See Trendyol.
Summary
See system integration & API or contact us.