Learn all about Korean culture while you learn the language! The two go together hand in hand; culture influences language and vice versa so it’s key to learn if you want to really understand Korean.
Now Koreans don’t solely use these terms for people related to them, they are used freely! People call the woman running the restaurant 이모, boys call their older friends 형, etc. These terms are used to show how close and familiar you are with others (and for the ‘aunt’ at your favourite K-BBQ to warm up to her and maybe get extra side dishes haha).
A lot of Korean texting slang has a rooted meaning in the original word, just shortened to make it easier to type. Some originate from Korean itself like 엉 which is the most casual way to say yes or yeah in Korean, being reduced down to just ㅇㅇ. Or ᄋᄃ from 어디 (where), ᄆᄎ from 미쳤어 (crazy), etc.