To change an Phone video to 1280 × 720 (720p HD) in Shotcut while maintaining its original quality, you need to match your project settings to the Phone's specific frame rate.
Phone videos usually record at 30 fps or 60 fps.
Follow these steps to set the correct size and prevent pixelation:

Step 1: Set the 1280 × 720 CanvasSetting the project resolution before adding your video prevents Shotcut from distorting the image.

Open Shotcut and look at the New Project panel.
Click the Video Mode dropdown menu.
Goto Other and select either HD 720p 30fps or HD 720p 60fps.

Tip: If you do not know your Phone video's frame rate, choose 30fps for normal videos or 60fps if it was shot in smooth/action mode.Name your project and click Start.

Step 2: Bring in Your Phone Video.

Because modern Phones shoot in a standard 16:9 widescreen format, a 720p project is the exact same shape as your video—just a lower pixel count.
Drag your Phone video into the Timeline.
At the top above your video select Filters.
Click on the plus icon.
Add Size, Position & Rotate.
Change the size to 1280 x Height leave this to what ever it changes to after adding the 1280.
Play your video in the viewer and adjust the position as needed.

Step 3: Export with High-Quality Settings.

To make sure Shotcut does not compress your video too heavily during export, use these high-utility settings:
Click the Export button on the top toolbar.
Select the Default preset from the list on the left.
Click the Advanced button at the bottom of the export panel.
Under the Video tab, verify that the Resolution reads 1280 × 720 and the Aspect ratio reads 16 : 9.
Click the Codec tab and change these two settings to preserve quality:Rate control:
Set this to Quality-based VBR.Quality: Change this to 65% (this gives you a visually lossless file where the human eye cannot detect any compression artifacts).
Click Export File, choose your save destination, and save the video.