Copy files between your Mac and Android phone

Select: click a file or folder. ⌘-click adds one more, ⇧-click selects everything between the last click and this one, ⌘A selects all. Esc, or a click on empty space, clears the selection.
Navigate: ↑ and ↓ move the selection. Hold ⇧ while pressing them to select several. → or a double-click opens a folder. ← goes up one folder, and so does clicking a folder name in the path above the list.
Copy: select on one side, then click the button in the middle. Folders are copied with everything inside, except Mac files and folders whose names start with a dot. Only one side can have a selection, so the button always knows which way to copy.
Delete: the trash button at the top right of the pane, or the Delete key. It asks first, skips the Trash and cannot be undone.
Rename: on the phone side, select one item and press Enter, or click the pencil button. On the Mac, rename in Finder.
New folder: the folder button at the top right of each pane. The folder is created in the folder that pane is showing.

Over USB, right in Chrome.
Nothing to install, and no files are uploaded.

1Plug the phone into this Mac with a USB cable, and quit Preview, Photos and Image Capture — they can block the connection.
2Unlock the phone, tap the Charging this device via USB notification, and choose File transfer.
3Click below and pick your phone from the list.
4Choose the folder on this Mac the app may use. It is the only place on the Mac the app can see, so files go to and from it. Chrome refuses the Documents, Desktop, Downloads and home folders directly, so use or create a folder inside one, such as Documents/Phone.