I use Visual Studio Code, and this is what I do when I need to look for things like that.
1. Have a workspace with both VP repository and Civ5's Assets folder (I made a copy with only the .xml and .lua files).
2. Use the Search function and look for TECH_ASTRONOMY.
3a. You'll find a hardcoded variable in CvGame.cpp set to the Astronomy tech ID, referenced in CvGame::getOceanPassableTech(), but that one's unused.
3b. VP didn't modify the tech, so you'll find it in CIV5Technologies.xml from Expansion2 which has EmbarkedAllWaterPassage set to true.
4. Now search for the Domestic Focus promotion.
5. You'll find the text key in PromotionTextChanges.sql (TXT_KEY_PROMOTION_OCEAN_IMPASSABLE_ASTRO), which points to the PROMOTION_OCEAN_IMPASSABLE_UNTIL_ASTRONOMY promotion in OldPromotions.xml.
6. Search for the promotion and you'll find PromotionChanges.sql set PassableTech to TECH_COMPASS for the promotion in the UnitPromotions_Terrains table.