Agrego valor correspondiente de acuerdo al turno

src/component/board.py

 import PySimpleGUI as sg
+from itertools import cycle
 from src.windows import board
 
 
@@ -8,7 +9,13 @@ def start():
 
 
 def loop():
-    window = board.build("Fede", "Milka")
+    player_1 = {"name": "Maria", "value": "X"}
+    player_2 = {"name": "Marta", "value": "O"}
+    turn = cycle([player_1, player_2])
+
+    board_data = [[" "] * 3 for _i in range(3)]
+
+    window = board.build(player_1, player_2, board_data)
 
     while True:
         event, _values = window.read()
@@ -17,7 +24,7 @@ def loop():
             break
 
         if event.startswith("cell"):
-            _prefix, x, y = event.split("-")
-            print(f"Celda: {x},{y}")
+            player = next(turn)
+            window[event].update(player["value"])
 
     return window

src/windows/board.py

 import PySimpleGUI as sg
 
-def build(player1_name, player2_name):
+
+def build(player_1, player_2, board_data):
     layout = [
-        [sg.Text('Jugador 1: ' + player1_name, key='-P1-', text_color='darkblue')],
-        [sg.Text('Jugador 2: ' + player2_name, key='-P2-', text_color='white')],
-        [sg.Text('')]
+        [sg.Text("Jugador 1: " + player_1["name"], key="-P1-", text_color="darkblue")],
+        [sg.Text("Jugador 2: " + player_2["name"], key="-P2-", text_color="white")],
+        [sg.Text("")],
     ]
 
     for y in range(3):
         layout += [
-            [sg.Button(' ', size=(8, 4), key=f"cell-{x}-{y}") for x in range(3)]
+            [
+                sg.Button(board_data[x][y], size=(8, 4), key=f"cell-{x}-{y}")
+                for x in range(3)
+            ]
         ]
 
-    board = sg.Window('Ta Te Ti').Layout(layout)
+    board = sg.Window("Ta Te Ti").Layout(layout)
 
     return board
\ No newline at end of file