Meals and allergies
Choosing a meal, and who reads what you write.
If the couple has opened their menu, you pick for each person as part of your reply.
If your link does not ask about food, either they are not offering a choice or they have not settled the menu yet — a caterer's menu often arrives months after the invitation does. The same link starts asking when they are ready, so come back to it rather than looking for a new one.
Telling them about an allergy
There is a box for it when you reply. Be specific: “severe nut allergy, including traces” is far more use to a kitchen than “no nuts”.
It goes to the couple and the planner they have invited, and to nobody else — not to other guests, and not even to the rest of your own household. It is held separately from the rest of the guest list, with tighter rules about who can read it, because under UK law an allergy is health information.