Use an Insert operation to load data because the data is not already in Salesforce and does not need to be updated or deleted3. Map an External ID data value to the object because External IDs are used to create relationships between records that are being imported and records that already exist in Salesforce4.
References: 3 https://help.salesforce.com/s/articleView?id=sf.data_loader_insert.htm&type=5 4
https://help.salesforce.com/s/articleView?id=sf.data_loader_relationships.htm&type=5