@antasia1127 Saves are stored locally on your computer, like other user data. So you'll need to transfer it to your new computer. Everything you'd want is in the Sims 4 folder in Documents > Electronic Arts, so you can transfer the entire folder if you want, or just the saves and Tray (saved households and builds) folders inside. I would suggest zipping them before the transfer to protect the files.
You can use an external drive or USB stick, or a free filesharing service or cloud storage service, whatever you're comfortable using.