To install add-ons or complements in Kodi, there are two different options depending on whether they come from, a repository already available in the app or if you have downloaded the add-on from the Internet in ZIP format. We will explain to you how to do it in both cases.
Installing add-ons on Kodi for Android from repositories
In Kodi, there are different add-on repositories from where you can download and install those that interest you. First, open the app and click on the Add-ons entry in the left side menu.

Look at the same side menu, at the top, for an open box icon and tap it.

Now in the central area of the interface, you will find the option Install from the repository, which will grant you access to all available online repositories of add-ons.

In the following list, you can see the different categories of existing add-ons. Enter the category of the type of add-on you want to install, in this case, we will look for a video add-on.

Browse the list until you find the add-on you want and tap on it.

It will open the tab of the selected add-on where you will see screenshots, read information about the add-on and what really interests us, press the Install button in the lower right corner. Tap this button to start the installation of the add-on.

Several available add-ons require other add-ons to function properly. Tap Ok on the right side to install all add-ons required.

When the installation is complete, you will see a momentary notification in the upper right corner of the interface.

You can now return to the Home screen and access the Add-On section. There, you should see the icon of the add-on you just installed. You can access it by simply tapping on the icon.

Install add-ons on Kodi for Android with ZIP files
The second alternative for installing add-ons is to do so from a ZIP file downloaded from the Internet. There are add-ons that are not available in the online repositories and need to be previously downloaded.
The first thing to do is properly configure the user to be able to install this type of extension. Open Kodi, go to Home screen and tap the icon with the shape of a cogwheel on the top left of the screen, it will grant you access to the configuration, right on the side menu.

Enter the System section displayed in the lower right-hand corner.

Now in the menu on the left select Add-ons.

You must move the Unknown sources option switch to the right to allow the installation of add-ons from unknown sources.

Confirm the setting by pressing Yes in the pop-up window that appears on the screen.

Once you have made this adjustment, you can proceed with the installation of the add-on. Go to the Kodi Home screen and tap on the side menu the option Add-ons.

Now, find the open box icon at the top of the side menu.

Among all the options now available on screen, tap on Install from zip file.

Use the Kodi's file browser to locate the Android directory where you store the add-on. Once the ZIP file is selected, the installation is executed immediately.

After a few seconds, you will see a notification in the upper right corner that the add-on has been installed.

Just go to the Add-ons section on the Home page to see the icon of your new add-on and access its content.