Day 2: The Restaurant's Cooks Loves Metal

Starting with an article signals that the apostrophe s is on purpose.

YesterdayLast night, I went to a Japanese and Chinese fusion restaurant with my friends and my girlfriend.

Usually restaurants are either Japanese or Chinese. One doesn't typically expect to get sushi in the same place they order lo mein. If I was describing it verbally I'd say "Japanese slash Chinese restaurant". For this I added "fusion," which even if it isn't quite true, sets the stage better.

It's a really good restaurant the sushi was very delicious, but the restaurant was kind of informal so it's the type of restaurant thatbut kind of informal. Its definitely the type of place you visit when you want to have a good time with your family or friends. The sushi was quite delicious.

Typically "very" + "delicious" sounds awkward. "Quite delicious" or "super delicious" or incredibly delicious" are more natural phrasings. The conjunctive phrasing isn't necessary and its confusing to pair the informal with the good times with friends. No need to repeat "the restaurant" sevearl times.

Something that was a littleIt was a bit funny to me was that the restaurant's cooks had hardcore metal music playing in the kitchen, and. So whenever they opened the door you could hear Slipknot and Rammstein songs in the background.

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Yeah, that would be a pretty funny pairing with your sushi.

Meanwhile outside the kitchen (I mean the place where all the customers were eating their sushi and dumplings)In contrast, the main dining area was playing Taylor Swift, Lady Gaga and some K-pop songs in the background.

When I realized that, I smiled for some minutes hahaere were dueling music tastes, I smiled for quite awhile.

YesterdayLast night, I went to a Japanese and Chinese restaurant with my friends and my girlfriend.

Instead of "yesterday night", it's more natural to say "last night"

It's a really good restaurant and the sushi was very delicious, but the restaurant was kind of informal so iit kind of had casual vibes. It's the type of restaurant that you visit when you want to have a good time with your family or friends.

this is just my interpretation of your sentence, to make it sound smoother

Something that wasI found a little bit funny to me was that the restaurant's cookhefs had metal music playing in the kitchen, and when. Whenever they opened the door, you could hear Slipknot and Rammstein songs in the background.

"chefs" just feels more natural in this context

Meanwhile, outside the kitchen (I mean the place where all the customers were eating their sushi and dumplings) was playing, Taylor Swift, Lady Gaga and some K-pop songs were playing in the background.

When I realized all that, I smiled for somea couple of minutes haha.

Day 2: Restaurant's Cooks Loves Metal

It's a really good restaurant t. The sushi was very delicious, but the restaurant was kind of informal so it's the type of restaurant that you visit when you want to have a good time with your family or friends.

Meanwhile outside the kitchen (I mean the place where all the customers were eating their sushi and dumplings) was playing Taylor Swift, Lady Gaga and some K-pop songs in the background.

Outside of the kitchen = in the dining room.

When I realized that, I smiled for some minutes haha.

Grammatically fine, but to sound more natural, I would say "for several minutes." Using "some" that way sounds stilted (at least in my American dialect).