What will happen if I export a 720p video in Photos to 1080p?
You've only got so many 'real' pixels to play with, so when you stretch that out to 1080p, you've got quite a few holes.

From there it all comes down to what approach the software you are using takes to create the missing pixels.  Typically the stretch it out to the edges, and then interpolate back to fill the holes, based on what the two pixels look like either side of the gap.

Some software does this better than others, and the best method to use can depend on the image content also.  Typically it makes things look softer, blocker if bad, and highlights poor quality issues such as moire etc.

Premiere Pro includes a simple algorithm as standard, and you can use plugging more targeted to the task if you want also.
There's no right or wrong.  A full HD TV will be doing this 720p to 1080p conversion ever time you watch anything less than full HD anyway for example.

Here is a simple graphics showing what happens