This trophy is awarded for completing all locations in the game, including:
Viewpoints (not sure whether these are required for the trophy, but they reveal all locations in the vicinity so they make it a lot easier)
Enemy camps / hideouts / ships (red icon)
Treasure locations
Stone circles (x12)
Tombs and ancient mechanisms
Hermit locations (x5)
War elephants (x4)
Following the 1.10 update, locations you have completed will have a small gold tick next to the location icon on the map. Previously there was a glitch whereby if you completed any locations while playing as Aya during the Blade of the Goddess main story quest, the trophy would glitch. This has apparently been fixed by the 1.10 update but maybe best to play it safe and only complete locations as Bayek anyway.
The trophy cannot be obtained until you complete the main storyline as a final location is revealed upon completion. This is easily the most time consuming trophy in the game, but isn't particularly difficult other than the War Elephant battles - as long as you are at level 40 these are not too tough but may take you a few attempts.
Locations not revealed by viewpoints
Tombs and stone circles (glowing yellow question mark) are not revealed by viewpoints. They will appear on the in-game compass when you are within ~450m.
This includes Eesfet Oon-m'Aa Poo, an ancient mechanism in the Sphinx which requires you to have completed all 12 stone circles and the associated side quest, and requires 50 silica to activate.
There are several locations which may not appear on the map even if you've synced a nearby viewpoint. These include:
Hotep Cavern (SW Qattara Depression)
Abandoned Fishing Village (Faiyum lake edge, SW of Beached Trireme Camp and NE of Taweret Stone Circle)
Dionysias Wharf (Faiyum, near Sea of Sobek Anchorage)
Map areas without viewpoints
There are no viewpoints in Great Sand Sea (no locations), White Desert (no locations), Black Desert (1 hermit location [also grants "I Can See My House From Here" trophy], 1 tomb and 1 ancient mechanism in the SE) and Desheret Desert (1 tomb, 1 ancient mechanism in the S).
Other tips...
1. Be thorough collecting silica in the tombs as you will need it to complete the Ancient Mechanisms (5 per mechanism, 50 for the one in the Sphinx)
2. Some of the question marks can be hard to see on the map in mountainous areas, particularly in the W and NW areas of the map
3. Try to fully clear an area before moving on so you know you haven't missed anything
4. The triremes move around the ocean / lakes so easiest to check these off when you are near them to save you having to chase them later
5. Use Senu to scout locations, particularly the later camps with multiple captains / commanders and treasure locations, to make sure you are clear on all objectives
6. You need to find all the papyri, since these are 'treasure' locations. However, you do not need to solve the associated papyrus puzzles.