In fact, I was proud to be an employee at the world's most famous fast food restaurant that even my grandmother in Korea knows of.
I presume your grandma lives in South Korea. It sounds very starnge that your grandmother knowing the existence of McDonald's is a proof of its prominence, especially if she lives in an urban area. Then it'd be like saying, "hey, even my cousin who lives in NYC knows about McDonald's." If that's not the case, you have to specify.
The place that harbored joys and delights in my childhood and also held my exposure exposed me to harshness and reality.
That "and" between "childhood" and "also" should be removed.
During my first shift as cashier, the managers snapped at me when I timidly asked them if they could open the cash register because I gave the wrong amount of change, .t T he customers let out exasperated sighs when I couldn't locate the appropriate buttons to compute their order on the menu screen, and my senior coworkers rolled their eyes when I pleaded for someone to make an ice cream cone for me after my unsuccessful attempts at shaping the cone.
This is just a suggestion. You should definitely break this up into 2 or more sentences.
Having a loud voice and an infallible cheerfulness were the trademarks of being a McDonald's cashier.
I like the style of your writing. However, some sentences are too long and lack commas.
The topic is fine, and so is the structure.