There are four meals: Breakfast, Lunch, Dinner and Snacks. Every food you log goes into one of them. The app asks which one before you start searching rather than after you have found something, which catches people out until they know to expect it.
The quick way
Tap Add to on the meal card you want. Search and camera open with that meal already chosen, and everything you log from there goes to it. This is the right answer almost every time.
Pinning a meal as your focus
If you are logging several things into the same meal (a big dinner, or catching up on a day you forgot), pin it instead. Open the More menu, choose Log to meal, and pick one.
While a meal is pinned, the screen shows "Adding to Lunch" (or whichever you picked) and every food you log goes there without asking again. Tap the same control to change it, or to remove the focus entirely.
The pin is a convenience, not a setting. It does not survive as a preference and it changes nothing about the day itself.
Redirecting one food
You do not have to back out and start again if you picked the wrong meal. When you tap a search result, the panel that opens has a Log to control on it. Change the meal there and only that food moves.
Meals and dates together
When you are on a day other than today, the header names both: "Log to Lunch" followed by the date. You can see at a glance that you are filling in Thursday rather than this afternoon. This is the most common way to log something to the wrong place, and the header is there to catch it.
Saved meals have their own default
Each meal in your cookbook carries a "Usually eaten at" setting, which is the meal it drops into by default. You can override it every time you add it, and changing it on the saved meal changes only the default, never anything you already logged.
Which meal should something go in?
It makes no difference to your totals. Calories and macros are summed across the whole day regardless of which card they sit on, so the meals exist to help you read the day back, not to change the arithmetic. Snacks is the home for anything that does not fit the other three.