If you want to multi-select files to either share, download, edit, etc., hover over a file and click the box in the top right corner to select it - OR - drag your mouse over the screen to make your selection. After selecting the first file, an action bar will appear at the bottom of your window.
From that action bar, based on your workspace permissions, you can:
Add the files to another Collection
Download the selected files
Share the selected files
Delete the selected files
From the three-dot menu icon, you can
Edit the selected files
Add teams and/or users to collaborate on the selected files
Copy the file IDs of the selected files to your clipboard
Or Archive the selected files