How to recruit Explorers and reveal artifact dig sites
As mentioned above, you need to study the Natural History civic if you want to get more artifacts in Civilization 7. Once the civic is unlocked, you should be able to recruit/buy Explorers in your lands.
But, that’s not all: You still need a University (built during the Exploration Age) or a Museum (built during the Modern Age) in your empire. Once you have an Explorer, send the unit to these buildings and select the Research Artifacts action. This will automatically reveal excavation sites with artifacts from the Exploration Age.
Note 1: Later, you also need to study the Hegemony civic. This reveals additional dig sites that contain artifacts from the Antiquity Age.
Note 2: The dig sites revealed by Universities and Museums are continent-specific. That means you must have either of these buildings so that the Explorer can unveil artifact locations in those regions.