Julian, read this part
for SHOP display mechanic, head to display.cpp (line 375) in SHOP, the array of catalog aka stock is called currentStock, the data will be shopItem array shopItem, diplay itemName, display price, display stock
for INTRO aka title screen, just modify titleOutput and startMenu (line 11). also you can modify the SCENE to create username on createUsername (line 55)
for displaying inventory, go to displayInventory (line 74)
for modifying COMBAT UI, look out for these 5 thing in both game.cpp and display.cpp game.cpp: runCombat (line 75), focus on the switch case action, make sure it return the action by modifying the combatMenu() from display.cpp (line 571). Try to use the entityStat of player attribute and entityStat of enemy attribute
for stages and displaying, all in explorationDisplay (line 201)