After applying on the platforms, you get invited to an apartment viewing. There are two major kinds.
PrivateThey invite you to a private viewing where you are allocated 10 to 15 minutes. This can be just you, or with a limited set of other applicants (2 to 3). Private viewing gives you the best shot at making an impression. We got a private viewing.
PublicYou and 100 other applicants show up to an apartment viewing. You queue up outside the apartment and go inside in a bunch.
If you're happy with the apartment and you've clarified your questions with the agent present there, you can ask how you can apply. It is a good idea to submit the documents there if you have it, including the application form. If not, you can ask if you can send it via email.