This is normal. When editing (except when doing lossless trimming ala Properties > Extract Sub-clip) the audio and video are re-encoded, which loses a generation of quality. You can export using a lossless or intermediate preset to lose no or nearly no quality, but I doubt you are going to like the file size. If you have a lot of drive space, give it a try.,
There is your problem. It did that because your project is set to 720p. That is related to
I exported it at 1080p with 30fps with a 16:9 aspect ratio.
When you increase the resolution in Export > Video you are not changing the project resolution. All images get converted to project resolution and then export resolution & aspect ratio if it is different.