Hi @RainKiko and welcome to the forum :)
The pack adds functions to the regular game, and adds a new world Copperdale. Once you have the pack downloaded, start up the Sims 4 game like you used to before installing the pack. You can play a household you were already playing, or another household, or play one of the households in Copperdale, or create a new household to live there, etc. as you like. You'll notice new things in the game like the Social Bunny and Trendi apps on the phone, Facial Cleanser available as an option on mirror and sink, Pillow Fight available as an option on a bed, etc.
If you have a teen in the household, when it's time for them to go to school (8am on school days) you'll get a pop-up asking if you want to join them at school or send them alone. Joining them will let you play them through their school day, at the high school. The school gameplay is locked to that one particular lot in Copperdale, but teens who live in any world can go to that school no problem. You can modify the school building if you want, by going to Manage Worlds > Copperdale > high school lot > build.