You can do calculations by hand or use brewing software. Choose a style that you want to brew. Generally, recipes have a significant amount of lighter malt. Brewers add specialty grains to get the color and malty flavor they want, as described in the style. They keep tweaking this until they have the right ABV and color. Both of these things tend to have wide ranges, so it's pretty easy to plan for a recipe to hit those targets. Then, people plan to add their bittering hops to get to a target IBU. Then they add flavor and aroma hops to add the appropriate taste and smell.
There are a lot of options along the way.
A given type of extract usually gives a certain amount of "points" per pound per gallon. For example Pale Dry Malt Extract will give 44 points per pound per gallon. Dissolving 1 lb in 1 gallon will yield a solution with an OG of 1.044. Take that value, multiply it by the weight you're using and divide it by the the total volume. That's how much it contributes to your OG.
With grain, you can't just say that you'll get 44 points out of a 1lb of Pale 2-row (that number isn't even correct, as Dry Malt Extract has more carbohydrates than grain). The mashing process will not get all of the carbohydrates into your wort. You can calculate an expected gravity value and compare it to a measured gravity at the end of sparging to get your mash efficiency, which should be between 75% - 85%, where 85% is really good.
People will measure the efficiency of their process and assume that value when making recipes.
One of the other thing that affects ABV is how much of it is actually fermentable. Malty beers taste malty because some of those carbohydrates weren't converted into fermentable sugars.
Finally, the yeast you use plays a critical role in determining your ABV, as it makes beer. Different strains of yeast have different attenuation rates, which is how much it will convert. Furthermore, yeasts have different alcohol tolerances. For high gravity beers, you may use more tolerant strains. Different amounts of yeast and how old it is also affects the beer.