When am I counted as joined after paying online?

When your payment is confirmed, not when you open the payment screen.

Paying by card, Apple Pay, Google Pay or Telegram holds your slot while you finish the payment, so nobody takes it while you confirm with your bank. Until the money goes through you are not announced anywhere: the organizer gets no notification, your name does not appear in the activity feed or in the event chat, and no calendar entry is created. All of that happens the moment the payment is confirmed.

Paying from your Playbox wallet, paying cash, and joining a free game all count immediately, because there is nothing to wait for.

If you close the payment screen without finishing, the held slot is released and you can start again from scratch.