Hi Janet,
The installer for AutoCAD LT for Mac should appear in Downloads folder by default.
The name of installer should be "Autodesk_AutoCAD_LT_2021_macOS.dmg", actually it is compressed installation image.
On next step you need to double-click on it in Finder, then you should see the contents of installation image in new Finder window:
Now double-click on "Install Autodesk AutoCAD LT 2021 for Mac.app" to start installation process.
Then you will be asked for your Mac Administrator account password, after this - new window should appear with "Install" button on it.
Click the “Install” button and wait for the installation to complete.