Inside, you’ll find a package file. Run it to "install" the installer into your Applications folder.
Apple officially provides as DMG files, which are used to update an existing Yosemite installation to the final version (10.10.5). These are not full installers. mac os 10.10 download dmg
Believe it or not, Apple still hosts the installer on their servers, but they hide the link deep within their support pages. You cannot simply search for it in the App Store unless you have previously "purchased" it. Inside, you’ll find a package file
Apple moved away from "skeuomorphism" (textures like leather and paper) to a translucent, flat design. These are not full installers
Yosemite no longer receives security updates from Apple. It is vulnerable to modern exploits. Only use it if you are disconnecting the Mac from the internet or if you require specific legacy software that cannot run elsewhere.