简单python入门函数

一个关于函数的小问题，但是没弄明白这个是怎么弄出来的我是初学者

def bonus(sales):
    if sales <= 10000:
        s = 5000
    elif 10000 < sales < 20000:
        s = 5000 + sales * 0.1
    elif 20000 < sales < 50000:
        s = 5000 + sales * 0.15
    elif 50000 < sales < 100000:
        s = 5000 + sales * 0.2        
    else:
        s = 5000 +sales * 0.25
    return s

sales =int(input(">>>"))
res = bonus(sales)
print(f"{res:.2f}")