diff --git a/app_src/tests/test_download_apk_screen.py b/app_src/tests/test_download_apk_screen.py
index 24f13e1..3e86b3a 100644
--- a/app_src/tests/test_download_apk_screen.py
+++ b/app_src/tests/test_download_apk_screen.py
@@ -46,6 +46,12 @@ def test_apk_is_valid_rejects_wrong_size(tmp_path):
 
 class _FakeResponse:
     def __init__(self, content):
+        """
+        Initialize a response with content and its byte length.
+        
+        Parameters:
+            content: The response content.
+        """
         self.content = content
         self.headers = {"content-length": str(len(content))}
 
@@ -53,11 +59,22 @@ def raise_for_status(self):
         return None
 
     def iter_content(self, _):
+        """Yield the response content as a single chunk."""
         yield self.content
 
 
 @pytest.fixture
 def fake_download_dir(tmp_path, monkeypatch):
+    """
+    Configure APK downloads to use a temporary directory.
+    
+    Parameters:
+    	tmp_path: Temporary directory used as the APK download location.
+    	monkeypatch: Fixture used to replace the APK directory resolver.
+    
+    Returns:
+    	The configured temporary directory.
+    """
     monkeypatch.setattr(d, "get_apk_directory", lambda: str(tmp_path))
     return tmp_path
 
@@ -106,6 +123,7 @@ def unbind(self, *_, **__):
 
 class _FakeUpdateButton:
     def __init__(self):
+        """Initialize the fake button with an unclicked state and a fake streak."""
         self.clicked = False
         self.streak = _FakeStreak()
 
@@ -118,6 +136,7 @@ class _FakeLaterButton:
 
 
 def _instantiate_screen():
+    """Create a download screen with mocked application dependencies for testing."""
     from kivy.event import EventDispatcher
     from kivy.properties import StringProperty
     from kivymd.app import MDApp
@@ -148,9 +167,17 @@ def test_start_download_uses_versioned_url_and_filename():
 
     class _FakeThread:
         def __init__(self, target=None, daemon=None):
+            """
+            Capture the callable provided as the thread target.
+            
+            Parameters:
+                target (callable, optional): Callable assigned as the thread target.
+                daemon (bool, optional): Thread daemon setting.
+            """
             captured["target"] = target
 
         def start(self):
+            """Run the captured target function."""
             captured["target"]()
 
     with mock.patch("threading.Thread", _FakeThread), \
diff --git a/app_src/ui/screens/download_apk_screen.py b/app_src/ui/screens/download_apk_screen.py
index 0f8ddd0..b263f9a 100644
--- a/app_src/ui/screens/download_apk_screen.py
+++ b/app_src/ui/screens/download_apk_screen.py
@@ -376,7 +376,12 @@ def add_body_to_new_stuff_container(self, markup_text):
 
     def start_download(self,_=None):
        #p("Clicked start download...")
-        if not self.built_ui:
+        """
+       Start downloading the update APK or switch to installation when a valid cached APK is available.
+       
+       The download runs in a background thread and updates the progress button as data arrives.
+       """
+       if not self.built_ui:
             return
         import threading
 
@@ -390,6 +395,9 @@ def progress(percent):
                 self.clock_for_progress_update.cancel()
             self.clock_for_progress_update = Clock.schedule_once(lambda dt: self.update_button.update_progress(percent))
         def worker():
+            """
+            Download the selected release APK and switch the download control to installation mode when successful.
+            """
             filename = get_apk_filename(self.new_version)
             apk_path__ = download_apk(
                 get_apk_download_url(self.new_version),
@@ -526,7 +534,16 @@ def do_not_go_to_update_screen(msg):
         Clock.schedule_once(lambda dt, e=e: do_not_go_to_update_screen(f"Failed:{e}"))
 
 def get_release_note_txt(data,latest_version):
-    """Check GitHub latest release version"""
+    """
+    Retrieve release notes for a specific version from the release assets.
+    
+    Parameters:
+        data (dict): GitHub release metadata containing an ``assets`` collection.
+        latest_version (str): Version used to identify the release-notes asset.
+    
+    Returns:
+        str: Downloaded release notes, or default release notes when the asset is unavailable or cannot be downloaded.
+    """
     import time
     import traceback
     import requests
@@ -557,13 +574,40 @@ def get_release_note_txt(data,latest_version):
     return release_notes or DEFAULT_RELEASE_NOTE
 
 def get_apk_filename(version):
+    """
+    Build the filename for a versioned APK.
+    
+    Parameters:
+    	version: The APK version identifier.
+    
+    Returns:
+    	str: The versioned APK filename.
+    """
     return f"waller-v{version}.apk"
 
 def get_apk_download_url(version):
+    """
+    Build the GitHub release URL for a versioned APK.
+    
+    Parameters:
+    	version: The APK release version.
+    
+    Returns:
+    	str: The download URL for the versioned APK.
+    """
     filename = get_apk_filename(version)
     return f"https://github.com/Fector101/wallpaper-carousel/releases/download/v{version}/{filename}"
 
 def get_apk_size(data):
+    """
+    Finds the size of the first APK asset in release metadata.
+    
+    Parameters:
+    	data (dict): Release metadata containing an `assets` collection.
+    
+    Returns:
+    	int: The APK file size in bytes, or `0` if no APK asset is found.
+    """
     for asset in data["assets"]:
         if asset["name"].endswith(".apk"):
             size = asset["size"]