Most professional Fighters and even most amateurs nowadays use the water loading, and strategic dehydration method. There’s a lot of different ways to achieve the same goal, and a lot of components in doing so. So to save time I’m going to illustrate you an example
Say you walk around 175. You have an upcoming fight that’s at 155. The fight is on Saturday and the weigh-ins are on Friday. From Sunday until weigh-ins on Friday you eat minimal carbs and salt. And no full meals, but eat protein. Sunday you drink 2 gallons of water, Monday and Tuesday you drink 1 gallon of water, Wednesday I half gallon and Thursday or quarter gallon. Friday you eat and drink nothing until weigh-ins for safe measure.
This goes on top of working out so that you can lose the weight. And most Friday’s wear multiple layers of clothing, you sauna suits or go to the sauna to help shed pounds during fight week.
There’s also different diets you can use if you’re a vegetarian which will allowed to eat more carbs. But I hope you get the idea