You can access and start installed and authorized add-ons directly from the application the add-on extends (Gmail, Docs, Sheets, Slides, or Forms).
Starting a G Suite add-on
G Suite add-ons that you have installed automatically appear in the host application interface as a column of icons (under icons for Google products like Calendar, Keep, and Tasks). The host application represents each add-on by a small icon; this icon is specified in the add-on's manifest.
Clicking an add-on icon opens the add-on interface, and clicking the icon again hides the interface. The location of the icon depends on the client (desktop or mobile).
An icon for each G Suite add-on a user has installed appears in the right-nav of the G Suite host application. An icon is only presented if that G Suite add-on has been enabled for use with that host in its manifest.
When a user clicks a G Suite add-on icon, a corresponding trigger fires to build the initial homepage for the add-on. Once the homepage card is constructed, the add-on returns it to the host application for display. If the add-on doesn't define a homepage, a default card is presented instead.
If you open an add-on that isn't fully authorized, it prompts you to authorize it. After you review and authorize the requested permissions, the add-on homepage appears, and you can then begin interacting with it.
You can close the add-on UI at any time by clicking the add-on icon again (or clicking the icon of a different add-on), or by clicking theicon in the add-on header.
G Suite add-ons can extend the Gmail compose UI, which provide add-on functionality from Gmail's "compose message" window. For these add-ons, the add-on icon appears in the horizontal row at the bottom of the compose window. The add-on icons are placed to the right of the standard icons for message formatting, attaching files, and so forth.
On mobile (Gmail only)
On mobile, the G Suite add-ons that extend Gmail have icons that appear as a horizontal row at the bottom of the currently open message or draft. Clicking an icon opens that interface at the bottom of the message.
For G Suite add-ons that extend the Gmail with compose actions, the add-on icons appear in the compose view's top-right menu.
Starting an editor add-on
Most editor add-ons create one or more menu items in the Add-ons menu of the editor. It's a best practice to create at least one menu item for an editor add-on to serve as the initial launch point that explains the add-on's purpose and usage. Selecting one of the add-on menu items causes some functionality of that add-on to start; often add-ons use menu items to open the main sidebar or dialog that represents the primary add-on interface.
Google Forms doesn't use the same menu bar as the other editors. Instead, you can find Forms add-on menu items by clicking theicon in the upper left of the Forms editor interface. Forms add-ons only extend the form editor, not the forms that are sent out to for replies.