App Lab is a distribution channel for Quest apps that exists alongside the official Store. Unlike apps and games on the main store titles submitted through App Lab are not showing up in the Quest Store tab. You won't find them in Oculus mobile app either. The only way to access App Lab Apps is by direct link. They are unlisted. This is why this and many other sites exist: to provide a list of these "unlisted" apps.
No. Using App Lab apps is save for your VR headset. Although requirements for App Lab are not as strict as in the main store, all apps still go through a review process.
No. You don't need a developer account to access App Lab. Just click a link and it will open a store page.
You can find all App Lab apps on this site: VR Sort or r/applab or App Lab DB