How does exporting affect the original quality of videos?

Yes. If you export a movie shot at 720 as an 1080 one, then the same data is stretched over more pixels, and consequently risks looking worse. How much worse? It's impossible to say, as it depends on the quality of the original video, the subject of the video and so on. It's all trial and error.

File>Export>Export Unmodified Originals.

You know, you're going to look at these on a monitor of some sort, and you're going to expect them to look the same size, so the size change is going to happen. If you don't change the size now, the viewing system will do it later. Don't worry about it. At all.

Okay, cause I exported a video that is 720, both to 720 and 1080, and tried to compare those two. And I think the 1080 became worse, but I don’t know if I’m tripping or not

You're not. As Yer_Man noted, you're scaling the video up to larger size. It has to fill in those pixels from somewhere. And that somewhere is, "from thin air".

It's no different than scaling up a still image in Photoshop, Preview or whatever. It has to add more pixels from nothing, which makes the image softer. And if it's a big difference, pixelated and soft. Such as below, left to right: 100% looks decent. 300%, ugly. 800%, hopelessly ugly.

And you're fighting compression from the get go. Videos from your phone are compressed. Which means you already have at least some detail loss. BTW, this is necessary. If you were able to shoot uncompressed video, even on a phone with 512 GB of space, you'd fill it with about 40-60 minutes of video.

So you start with compressed video, then when you export your project, it gets compressed again.

So it’s just the size? Not the quality?

It's both. Any time you scale digital assets up, you're going to degrade the image. It's unavoidable. Small amounts you won't notice, but 720 to 1080 requires 34% more pixels than exist in the 720 footage. That's a 150% increase in size, which will be noticeable.

If you're editing these videos individually, then work on them at their native size. If you're mixing the sources, then it would be better to build the composite video at 720. Scaling down isn't anywhere near as bad as scaling up. It will soften the 1080 clips a bit to scale them down, but then just add a sharpening filter to those clips.

DON'T go overboard with the sharpening. Here's an example of what your sharpened clips should not look like.

The main thing to watch for is you should never, ever see halos on anything. Nothing in the real world has halos. Even the original is too sharp as you can see a halo between the top right of the object and the sky. The sharpened image above is already way overdone.

Your enemy in unsharp masking is the radius.

Original:

High sharpening with a low radius sharpens the image without creating a halo.

Same amount of sharpening with too much radius.

Format is rather irrelevant. You have to bring in what you have, and both are already compressed. When you export your composite video, you can choose .mp4, which is still a compressed video format. The original compression then gets compressed again, which means at least a bit more detail loss.

As Yer_Man suggests, the only way to know for sure is to try a few and see. I would guess that in most cases, with videos of pretty normal things, that the conversion is pretty good, and you won't be able to tell the difference. Avant-Garde videos with strange swimming colors and swirling lines might not convert as well-- the algorithms are designed to work best with most subjects.

But, as Yer_Man says, try it and see!

You can choose File>Export>Export Unmodified Originals. Then that's exactly what you get. If you hadn't made edits or comments that you want to keep, then this is ideal.

Drag and drop is not really supported. It may do what you want, or it may do something else. As OSs have changed, drag & drop has produced originals or thumbnails or previews or edited jpgs. If I don't care much what I get (like sending a small picture in a Message) then I drag & drop. Otherwise I Export.

Pictures are not files until you choose Export and define the type of file you want. The edited pictures never exist as edited picture files. Photos is a non-destructive editor. Photos records the steps you take to edit a picture, and then it applies those steps on the fly when you want to look at it or you export it. There is no file to change when you choose File>Export.

So it’s actually the size, but then it affects the quality as well?

If you scale the image, then you will affect the quality. Native size is always best.

Or is it literally both size and quality? For example, 1080 stands for both better quality and size?

Only if it's already 1080. Any other size would have to be scaled (adding or removing pixels to make a non-native 1080 video fit that dimension).

But like said, some videos are in 720 and some in 1080 and if I export I have to choose 720 or 1080 for all of them.

Then separate them by size. Export all of the 720 originals to 720. Then all of the 1080 originals to 1080.