If you'd like to save your files from Collect to your Android phone, you'll just need to do the following:
Press and hold down on any file in an open board, your Sort later, or your All items and you will see the option to select each image you want to save.
After choosing which files you want to save, tap the options button in the top-right corner of the screen (that's the button with the 3 dots).
Then choose the option to 'Save to device'.
The app creates a folder called 'WeTransfer' to store the files downloaded off the app. If the saved files are images, they'll be saved to your phone in Pictures under a WeTransfer folder. If the saved files are videos, you should be able to find them in a WeTransfer folder in the Movies folder of your device.
That's all there is to it! Your file will now be saved to your device and you can do whatever you like with it.