Short answer

Drag the file from a Finder window and drop it into the other app's window, or onto its icon in the Dock. Put both windows side by side first with Window ▸ Move & Resize. In the Finder you can also copy a file with ⌘ C and press ⌥ ⌘ V in another folder to move it.

Most Mac apps accept files you drag into them. Mail turns them into attachments, chat apps send them, editors open or insert them. The tricky part is usually getting both windows on screen at once. These methods cover that and a few ways around it.

Drag and drop between windows

In the Finder, select the file. Hold ⌘ and click to select more than one.
Press and hold the trackpad or mouse, then drag the files over the other app's window.
Let go where you want them, such as the body of an email or a chat box.

When you drop a file into another app, your original usually stays where it was in the Finder.

Dragging between Finder folders works a little differently:

To keep the original and put a copy in another folder, hold Option while you drag.
Dragging files to a different disk copies them. To move them instead, hold ⌘ while you drag.
Hold Option and ⌘ together to make an alias instead of a copy.

Put windows side by side

Drag and drop is easiest when you can see both windows. Recent versions of macOS can tile windows for you.

Click the first window, such as your Finder window.
Hold the pointer over the green button in its top-left corner. A menu of tiling options appears.
Choose a left or right tile. You can also choose Window ▸ Move & Resize from the menu bar.
Do the same for the other app's window, choosing the other side.
Drag your file across.

If you want both apps to fill the screen with nothing else around, choose Window ▸ Full Screen Tile instead.

Use Stage Manager

Stage Manager keeps the app you're using in the middle and lines up your other apps on the left. It suits people who don't like arranging windows by hand.

Click Control Center in the menu bar, then click Stage Manager to turn it on. You can also use System Settings ▸ Desktop & Dock.
Drag an app from the left side of the screen into the center. It joins the current app as a group, so both are visible.
Drag your file from one window to the other.
To split them up again, drag the app back to the left side.

Drop a file on an app in the Dock

You don't always need the app's window open. If the app is in your Dock, drag the file over its icon and let go. The app opens the file. For example, drag a document onto the Pages icon to open it in Pages.

Copy and paste files

Many people don't know that files can be copied and pasted just like text. It is handy when the destination is a long way from the source.

In the Finder, select the files and press ⌘ C.
Open the folder where you want them.
Press ⌘ V to paste a copy. Or press ⌥ ⌘ V to move the files there, removing them from their old spot.

The Finder has no Cut command for files. ⌥ ⌘ V does that job.

When the goal is to send a file rather than drop it somewhere, the Share menu skips the window juggling.

Select one or more files in the Finder.
Click the Share button in the toolbar. For items on the desktop, Control-click the item and choose Share.
Choose how to send it, such as AirDrop, Mail or Messages. The options depend on the type of file.

Carry files on a Shelf with FileShelf

Sometimes the two windows never fit together, or you gather files from several folders before you send them. FileShelf adds a Shelf for that. While you drag files, shake the pointer or push it against an edge of the screen, and a Shelf appears to hold them. Switch to the other app at your own pace, then drag the whole stack from the Shelf into it.

The Shelf also offers optional Email Size and Zip actions for getting files ready before you send them.

Park files on a Shelf, switch apps, then drag them all in at once.

7 days free, then pay once.

Download FileShelf

How to move files between apps on Mac