How can external applications integrate with Salesforce?

External applications can integrate with Salesforce primarily through the use of Apex Web Services. This approach allows developers to expose Salesforce’s business logic and data to external applications using web service protocols. By creating an Apex class and annotating it with the appropriate web service annotations, developers can enable external systems to call these services, allowing for real-time data exchange and interaction with Salesforce records.

Apex Web Services support both SOAP and REST protocols, providing flexibility in how external applications can interact with Salesforce. This capability is crucial for applications that need to perform operations like creating, reading, updating, or deleting records in Salesforce from another system.

The other options, while relevant to certain integration scenarios, do not play the same foundational role. For instance, Salesforce Connect is used primarily for connecting external data sources directly and does not entail building custom API services. Apex triggers respond to changes in Salesforce data rather than facilitating an external application’s direct access to Salesforce. Custom Visualforce pages are primarily for UI presentation and do not directly facilitate data integration from external applications.