What's the difference between "Oh my God" and "Oh my goodness"? Does the term OMG also refer to "Oh my goodness" as well?

What does "underdressed" mean? When can I use this expression?

"Underdressed" refers to being dressed too informally for an occasion. For example, in many cases, attending a wedding wearing jeans, a t-shirt and sneakers would be considered "underdressed". You can use "underdressed" to describe someone who is not dressed nice enough for a role or event. Ex: I feel underdressed, everyone at this party is wearing a dress, and I am wearing sweatpants. Ex: He is underdressed for his new job, he is wearing a t-shirt when he should be wearing a dress shirt.

In what situation do you use the phrase "like I was saying..."?

Great question. "Like I was saying" has two uses. It can be used when you are saying something again that you have already said or, it can be said when you wants to return to a previous topic of discussion. The first form is appropriate in any conversation. However the used of "like" is casual, informal, and familiar. In a more formal situation you may hear people, this use "as I was saying" instead. Ex: Like I was saying before the waiter came over, I'm going to visit my mom next month! Ex: Thank you, James. Now, like I was saying, you'll need to call payroll to get those reports. Ex: It’s unfortunate but, like I said, it’s a decision we have to make.

Would it sounds awkward if I say "practice" instead oof "do simulation"?

"Simulation" is a more precise word meaning to imitate a hypothetical situation. "Practice" could be used instead of "simulation", but it is a more general term meaning to repeat something to improve it. Using "practice" over "simulation" depends on how precise you want to be.

What's "hold on" mean and when's it used?

"Hold on" is a way of telling someone to wait. One can also say, "hold on a second" or "hold on a moment." "Hold" in this context means to refrain from doing something else, not to continue what they were doing. It can be used when you need someone to wait a bit, usually when you have to do something or check something, but it isn't usually used for long periods of waiting, just short ones. Or, it's used to draw attention to someone or something so that everyone stops doing something. Ex: Hold on, I need to tie my shoelaces. Ex: Just, hold on. I'll be back in five minutes. Ex: Hold on! Where are we going, and why?

What does "swirl" mean here? Is it related to the appearance of the ice cream?

A "swirl" is a twisting shape, mark, or pattern. In this case, it refers to the mix of two different flavor ice cream, which is usually mixed today in a swirl pattern.