The assets used for company branding need to be uploaded to the AzureAD B2C tenant via the Entra Admin Portal GUI. Then, any component that references a branded asset, such as a html template used for a given self asserted page in custom policy, will display the custom branding.
You will now find that your company branding shows up on B2C self asserted pages.
PSA: due to caching on the servers which serve custom policy, changes to custom policy, assets, etc. can take anywhere from 30 minutes to 1 hour. If you have your custom policy set deployed in dev mode, this time will be cut down to roughly ~ 5 minutes.
The custom policy starter pack Microsoft provides will automatically show this branding without any customizations should you follow the tutorial.
If you are still having issues after uploading your branding to the AzureAD B2C tenant I recommend implementing the starter pack as a separate policy set, observe your branding properly rendered on a self asserted page, then reverse engineer/compare/contrast the starter pack custom policy with your own to fill in the gaps.
As a final reminder, ensure you are actually "in" your AzureAD B2C tenant when uploading your branding assets. For admins who use a single account to hop between multiple tenants... they sometimes (myself included) get mixed up what context they are in. You can change tenants using the tenant switcher here.
