Why does the game run at 60fps?

I was looking through the core rpg files, but I couldn't find anywhere where 60fps was defined as the target frame rate.

Is it somewhere that I missed? Or is it somewhere in PIXI? Or maybe it's built into the browser?

If the main loop is executed in less than 1/60 of a second, what happens to the remaining time - does the browser just do nothing until the next main loop?

If I recall correctly the game calculates the time between frames, and if it doesn't add up to a certain amount, it waits the next frame to render. It should all be in SceneManager.

I think it runs at the Hz of your monitor. So 60Hz = 60fps. I don't know if it still does, but if you ran your monitor at a lower/higher Hz some logic would be ruined since the engine measures things in frames instead of, for instance, milliseconds/seconds.
I believe browsers also always run at the Hz of your monitor.

There is a plugin called Fluid Time Step that fixes this and I would heavily recommend using it.

Being curious about this topic, I took a look at the Fluid Time Step plugin (https://forums.rpgmakerweb.com/index.php?threads/fluidtimestep-fix-the-game-speed.50709/) but the author had a note saying that if you're using MV 1.1.0 or newer, you don't need this plugin because apparently MV incorporated the code in rpg_managers.js.

This may end up being a question for another thread but does MV (without the Fluid Time Step plugin) still have problems now with monitors with refresh rates higher than 60 Hz?

try add this argument to your app manifest
You game will run at 1000 FPS lol.
So yes , i think it your monitor thats control the game fps, and maybe your app configuration...

I guess not! Good to hear they fixed it.

Even if it displays more than 60hz, the game will run at 60hz. To be more exact, the game will render at 60hz even if the engine refreshes more than 60 hz per second. I remember I had a problem with the play time which use the frame count of the game which updated with the engine and not per render, creating higher play time than actual play time. That is on version 1.4 (since that's the version my game is running on), they probably fixed it.