The Android developer Settings guide is the best place to start before contributing to Chromium for Android's settings.
Many common utility functions that are useful for developing settings screens in Chromium for Android can be found in //components/browser_ui/settings/android.
The widget subdirectory contains a number of extensions of AndroidX Preference classes that provide Chromium-specific behavior (like Managed preferences) or common Chromium UI components (like buttons).
The base Preference classes included in the AndroidX Preference library can also be used directly in Chromium for Android Settings screens.