There are game modes. Other thing which i also metion are mods

There are 4 game modes: survival (default, you live and die, you build and eat, etc) creative (youre basically a god, you can fly, build, and have access to creative blocks which are powerful) spectator (turns you into a ghost to see through walls and underground) and adventure which is survival but you can break or place blocks, only interact with them...

There are also difficulties (easy to hard, but in extreme if you die you become spectator mode and cant respawn) and game rules with different effects, like fire not spreading, day/night cycle stopping, mobs do not drop anything, etc...

A mod is a separate minecraft script that adds or modifies things already in minecraft. For example the one in this video is alex's mobs which adds a ton of mobs and items, with each having unique behaviors, which are not in the vanilla game