There are a lot of tools could download videos to you iPhone. If you want to copy videos from PC to iPhone directly, you are also able to try this
iPhone data transfer software.
Leawo iTransfer allows you to transfer files among iOS devices, iTunes and PCs with ease. With this software, you don't have to worry about the sync of iTunes and the sizes of your video files, you can transfer all the video files you need at one time. The following guidance will show you
how to transfer videos from PC to iPhone, and you can free download the software and have a try. Note that the trial version has a limit on the times of data transfers. Download
Leawo iTransfer '
Leawo iTransfer for Mac and install it.
1. Run Leawo iTransfer on your computer and connect iPhone to computer with the USB cable. The software will recognize your iPhone and automatically detect the contents in your iPhone. Then the software will show the libraries of your iPhone in the left sidebar.
2. Click "Movies" in the left sidebar, and the contents will be shown in the right part of the window. If your Movie Library is empty, the software will offer an "Add File" button in the middle of the window to help you add video files from computer. Or you can click "File > Add > Add File/Add Folder" at the upper right corner of the window to add video files from your computer.
3. After choosing files, the software will start transferring videos from PC to iPhone. The progress bar in the dialog will show you the percentage of the transfer process, and when the transfer is done, you will get the videos in the target folder.

For more detail, check:
How to Transfer Videos from PC to iPhone
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