Will I gain weight (fat)?

Stay away from cheese and anything that is white. The chicken sandwich and hamburger should be all right but don’t put mayo or cheese on them. You’d be better off with a sandwich if you just eat what’s inside and not eat the bread. Eat your fruits and vegetables! Stay away from sugar!
Be sure to get enough exercise to help maintain proper weight and muscle.

What ever you settle on make sure it’s something you can live with for the rest of your life. Otherwise it’s just a temporary fad diet that you’ll go off of and then gain the weight back.

Fast foods tend to be empty calories with salt and sugar thrown in. Salads or protein bowl are just okay.

IMO there is really nothing wrong with fast food. It’s just regular food.

Fast food has more calories per bite. If you control the calorie intake, you will be fine. You just won’t be as full afterwards.

More calories than what @RocketGuy?

Fasting, and then eating fast food is not a good way to maintain your weight, or your health. Most fast food is way to high in calories, saturated fat, salt, and sugar.

Eating healthy involves eating mostly whole foods, fruits and vegetables, lean protein, and a lot of fiber. You should aim to eat nutrient dense foods, rather than empty calories which are not very nutritious.

Most fast food is NOT nutrient dense, and is very high in calories. A home grilled chicken breast on a whole wheat bun, with lettuce and tomato is much healthier than a breaded, fried “chicken patty” (made from lots of different parts, and other ingredients pressed together) on a white hamburger bun, slathered in mayonnaise.

Here are some good guidelines for eating healthy, and maintaining a healthy weight, but be aware that you need to make lifelong good eating habits such as these. “Diets” do not promote long term health, or maintain healthy a weight.

https://www.cdc.gov/healthyweight/healthy_eating/index.html

For years I had an Egg McMcmuffin for breakfast every workday morning. It has an egg (a normal egg) for about 100 calories. It has a piece of American cheese (a normal piece of cheese,) also 100 calories. A tiny bit of Canadian bacon, maybe 50 calories, all on an English Muffin. A normal, baked English muffin also ~100 calories. So a 350 calorie breakfast. Not bad. And just as healthy as one you’d make at home with eggs and cheese and muffins you get from the grocery store.

Typically I would have a muffin + egg at home, so 200 calories. So not as tasty as a McMuffin, but saves me that many calories per meal. Calories add up.

So in one movie, Super Size Me, the guy got supersized if the worker asked. He took in a lot of calories and gained a lot of weight. But here is a case where a different guy at only McDonalds but kept track of his weight and exercise, and actually lost weight: https://www.independent.co.uk/life-style/weight-loss-diet-fat-mcdonalds-calories-nutrition-super-size-me-ryan-williams-exercise-a8632016.html

Once again showing that calories matter.

Well you can order a McMuffin without cheese or the bacon. Also 200 calories even though it’s from McDonald’s.
I’ve been a professional calorie counter since 1989.

When I hit 40, my weight became closely correlated to my calorie intake. That’s when I started counting. No more KFC 3x a week.

Weight has always been correlated with calorie (energy) intake minus energy expended. Most of us slow down around 40 but most don’t reduce calorie intake.

I had to choose between cutting calories and going shopping (for bigger clothes). I chose the former.