How do I automatically add all existing apps to my home screen?
Android launcher(s) is a dedicated app that serves as an UI to the underlying OS. There are tons of launchers available with features catering to the tastes of needs of users. If one needs features to be added to the existing (OEM or third party), they cannot be provided by yet another app because the launcher is so closely tied with the Android OS. Either the developer of the launcher, provides the requested feature or one uses a different launcher with that feature. 

Case in point is the requirement of OP to add all installed apps with the OEM Moto launcher (instead of having
 installed apps in the app drawer). OP is looking for the functionality of ABC launcher to be added to Moto launcher. This isn't possible for reasons explained.

Android users, usually, let pre and subsequently installed apps be in the app drawer and choose the apps, they access more frequently on home screen, organized in the manner of their choice. Launchers let you search for  apps, even when they are not on home screen. In the app drawer, they may be sorted alphabetically, or organized by functions (e.g.games) or user created panes (e.g. shopping /banking apps) etc. The possibilities are endless and there are launchers available for different needs. Nova launcher is famous for exactly this - letting the user customize practically each element of the launcher interface (the reason I use it)

tl:dr; You choose the launcher that works best for you