I'm planning to try and gain some weight. My plan is to prep a lot of burritos and freeze them. I think it'll be easy to add lot's of protein with beans and make them high-calorie. Since you just need to microwave them, they're pretty convenient to eat too. I'm not so muscular, though, so I can't say that this is a proven and tested method.

Processed carbs can be pretty useful because they're high in calorie but aren't really filling so like bread, pasta, etc

You can bulk with cheap food pretty easily. Like 2 pb&js can easily be 400 calories. I recommend eating a high calorie breakfast like oatmeal with coconut milk, nut butter, and fruit. You can add some protein powder to that too if you need

The sub gainit is dedicated to gaining weight and they have a sister sub specifically for recipes! I have some random high calories recipes not from the sub saved on my phone if you want links :)

I find meal prep is the best way to gain weight. Use cronometer.com or another nutrition tracker to design a week of balanced, high calorie meals, make them in batches, then just eat then on schedule. You can add snacks for extra calories when you're not too full.

Shakes, burritos, rice and beans, and grain bowls are all good formats for packing in calories. For snacks, peanut butter, nuts, seeds, and trail mix are a good source of fats.