By default, most zones will spawn monsters at the same level as your character D2R ladder items. For example, if you're a level 30 character, most of the enemies you'll face will be level 30. Some zones will feature higher-leveled enemies such as Strongholds and the Helltide, but you'll typically be fighting monsters of the same level as you.
A level 12 player can play with a level 70 party member and still deal damage to monsters, ensuring group play is almost always an option. This level scaling also means that you can embark to an entirely different region of Sanctuary and experience that content without fighting monsters significantly under or over your character's level. There are some level minimums set in certain regions we'll cover in the next section, but this holds true for nearly everything in the campaign and open world.
While level scaling is a universal constant in Diablo 4, you won't be fighting at-level monsters for your entire playthrough. Some content will scale the monsters to always be above you, notably Strongholds and Helltide. There are also some areas that have a level floor before you can experience them, such as all monsters in World Tier 4 being at least level 74 or higher.
Nearly all content in Diablo 4 will scale to your character's current level, but certain milestone activities like capstone dungeons and Uber Lilith are set to a specific level buy d2r items. World Tiers will also raise the level floor of all monsters in the game, so be sure your character is properly leveled and geared before increasing your World Tier.