A link that opens straight onto the invite. There is nothing to sign in to and nothing to download.
They see the title, that it's from you, the day and the time, where it is, your photo if you added one, and your note. Under that, a map and Directions if you gave an address, and how many are going. Whether they see names depends on Who sees the guest list: Guests who've answered, All guests, or Only you.
The answer is Yes or No, with Not sure yet as a quieter third choice. After a yes they can say how many are coming. Past the plus-ones you allowed, the answer is still taken and you see that it's more than you planned for.
After answering they can add the event to Apple or Google Calendar, change their answer, and invite someone if you allowed plus-ones or public replies.
If you turned on Show my number on invites in Settings, a Message link lets them text you.
The page loads nothing from anyone else. It has no advertising or tracking scripts, and it sets no cookie until they answer.