Is my data security at the apple store?

Apple does not perform upgrades like adding memory or storage (and in fact, most recent-model Mac notebooks have the RAM and SSDs soldered to the logic board such that they cannot be upgraded). Apple performs repairs only. The only rick to your data when you send your Mac in for repairs data loss – Apple may need to wipe your internal storage, or if they replace the logic board then they'll destroy the old one including the onboard storage. Make sure you have a backup before sending it in.

For security, you should be using FileVault regardless (e.g. if your notebook is stolen).

Like @neuroanatomist mentions if your computer is not encrypted, then anyone with physical access to the computer can access the data on the drive. A user account password does not protect data on an unencrypted computer.