After downloading, you'll have to unzip the contents of the Web Part folder.

Each Web Part will contain the following structure:

To install the Teams add-on, we will need the contents inside the spfx folder:

── spfx 📁
└── Office365
└── SP2019

Click on Office365 and inside, you'll see a zipped folder containing the Teams add-on, that will allow for the installation inside Microsoft Teams.

Note: The contents inside the BT_webpartname MSTeams.zip should not be extracted, as the installation will require the zipped solution to be uploaded.