Video Quality does not match output quality

Somehow you got a 640x360 sequence starting out, so ... how did you create that sequence?

With the Timeline panel 'active', click in it to get the blue line around it ... go to the menu Sequence, Sequence Settings, and change the framesize data to 1920x1080.

Neil

Sequence Settings are incredibly important in Premiere. Those are what determine the specs of your video (frame size, frame rate, pixel aspect ratio, etc.)

So if you create a sequence with a low resolution (as you have) and then scale everything down to fit that frame size, now your video is that size - in your case it's 640x360. When you export you are now upscaling your 640x360 video to 1920x1080, and obviously that's not going to look very good.

So, very important to understand what a sequence is and how it affects your video. It is your video.  

How did this happen? Likely one of a few ways:

1. You started a Sequence manually and put in the incorrect settings or based it off of a low res preset

2. You started your Sequence from a video clip with those specs

3. You started your Sequence from an audio clip, thereby telling Premiere to match the audio settings, but that you don't care what the video settings are

Always pay attention to how you are creating your sequence, and if you are starting it based on a piece of source media, make sure it's the source media that you want to define the specs of your video.

How do you fix it now? It's fixable, and relatively painless depending on your familiarity with the software and the type of work that you've done in your project.

First, you need to set the Sequence Settings to what you want them to be - the specs of your source media in this case: 1920x1080, 59.94, PAR of 1.0 (Your media is Variable Framerate, which is a whole other can of worms to go into, and that's why it isn't showing a standard framerate but the average of what the fluctuating framerate of the clip is.)

With your existing sequence you can correct these settings by going to the Sequence Settings menu:

Enter the correct settings and press O.K. You may get a warning about deleting preview files, which you can say O.K. to (you may not even have video previews, and if you did they are worthless, since your sequence settings were incorrect.)

Now you will notice that your clips all have a black border around them, because they are still scaled down to fit the previously small frame size. Now you are looking at your 640x360 scaled clips in a 1920x1080 sequence.

This is where you hit a crossroads on what to do next:

If you have No Animated Motion Properties on your clips: Then you can select all the clips, right click, choose Remove Attributes, and uncheck everything except for Motion, and click OK.

Your media that is 1920x1080 will have the scale reset to 100% and will now fit your 1920x1080 Sequence. You're done.

If you DO have Animated Motion Properties on your clips: Then you don't want to use the above method, as that will destroy the keyframed work that you've done. You'll want to add the Transform effect to the clips with the Motion animations and you can use the Scale of the Transform effect to scale the clips back up. As long as you put the Transform effect on the clip it will not be destructive. Do not put it on an Adjustment Layer and apply that to the whole timeline.

Once you figure out the correct scale for one clip to bring it back up to the correct size you can copy that Transform effect and paste it onto the other clips with motion animations - or you can just use this method in place of the other one and paste the effect onto everything in the timeline.

After this you should be good to export and have your source match the output.

Working with Phone Media: If you notice issues with playback, errors, audio desync, etc., that can all be a result of the source media you're working with, which I assume is from a phone. HEVC is a really challenging video codec to work with, and Variable Framerate is like poison for editing software. With that combination you can expect a pretty rough editing experience. The media you work with has a massive impact on performance and stability. 

To learn more about Video Codecs: https://blog.frame.io/2017/02/15/choose-the-right-codec/

To learn more about Variable Framerate: https://www.reddit.com/r/VideoEditing/wiki/faq/vfr/