How to install After Effects Scripts, Extensions, and Plug-ins without administrator access

Scripts

To locate your scripts folder, open general preferences and click the “Reveal Preferences in Finder” button.

macOS:
/Users/<username>/Library/Preferences/Adobe/After Effects/<version>/Scripts

Windows:
%appdata%\adobe\after effects\<version>\scripts\

If needed, create the ScriptUI Panels folder within it.

Put your scripts there, restart AE.
Alternatively, as of CC 2019, there are shortcuts for installing scripts from the UI.

see:
AE 2019 Feature Summary

Plug-ins

Create the following directory structure if it does not exist:

macOS:
/Users/<username>/Library/Application Support/Adobe/Common/Plug-ins/7.0/MediaCore

Copy your plugins there, restart AE.

Extensions

Create the following directory structure if it does not exist:

macOS:
/Users/<username>/Library/Application Support/Adobe/CEP/extensions

Windows:
%appdata%\adobe\cep\extensions\

Put your extensions there, restart AE.

If you have a zxp, it can be unpacked with the ZXP Installer from aescripts:

It will ask you for your administrator password, click cancel (maybe twice) and it will allow you to do a manual install. Point it at the directory above, restart AE.