I bought a 512GB Macbook pro, but why is the storage only 494GB?

The differences are in the space taken up by formatting of the drive and by the operating system & apps.

The drive capacity specifications for nearly all drives are the manufactured capacity of the drive ... but the formatted capacity is always somewhat less. How much less depends on the specific format of the disk. This has always been the case for all computers & drives, not just Macs. So, a 512GB drive, after formatting, will always have less capacity than its spec.

Second, the operating system & apps take up space on the drive. After installing the operating system & apps the available capacity will be significantly lower than the manufactured capacity of the drive. And since Macs come with macOS & certain Apple apps already installed, as soon as you turn it on and look at the available drive space, it will always be less than the manufactured capacity.

The 16GB difference in your case is about right for a new 512GB MBP.

Where do you think the macOS it’s installed ? Must be in the ssd drive and that takes a part of the ssd storage