You can move any files that you own from your own Google Drive folders into a Shared Drive. If the file is owned by someone else, you will have to ask them to move the file into the Shared Drive.
To move a file from a Google Drive folder into a Shared Drive:
Locate and select the file you wish to move
Choose More button along the top of the screen
Choose Move To
Choose Shared Drives
Click on the Shared Drive you wish to move the document into and continue clicking on each subfolder until the required folder is selected
Click on Move here
You can also move files that you own from any Google Drive folder into a Shared Drive folder using drag-and-drop. Select the file you wish to move, then drag it over the required Shared Drive folder on the left-hand side of the screen.
NB: To avoid confusion, any file within a Shared Drive will only exist in the Shared Drive. The same file cannot be in two different drives or folders at the same time e.g. a file can’t exist in both My Drive and a Shared Drive.