The default launcher of LG G5 doesn't come with an App Drawer and all your apps and widgets are placed in the home screen. But to add and bring back app drawer on LG G5, you can either use 3rd party launchers, or do as follows and get another official LG launcher, called the LG Home 4.0, on your LG G5:
1. Open the SmartWorld app on your LG G5. (SmartWorld is one of the default apps on your G5 which is kind of the LG's app store.
2. Look for this app in the search box: Home 4.0.
3. Now, all you have to do is to install this LG's launcher, which comes with an APP Drawer as well as so many other features.
Note: If you have difficulty, finding the Home Launcher, simply click this link on your LG G5: Download LG Home 4.0 for LG G5