There are several ways to unlock new characters in Super Smash Bros. Ultimate:
Play through the game's single-player mode: As you progress through the game's single-player mode, you will unlock new characters as you complete certain challenges or meet certain criteria.
Play through Classic mode: Each character has their own unique Classic mode route, which is a series of matches that you must complete in order to unlock the character.
Play through All-Star mode: All-Star mode is a series of matches against every character in the game. As you progress through the mode, you will unlock new characters as you defeat them in battle.
Play through World of Light mode: World of Light is a single-player adventure mode in which you explore a map and battle against a variety of opponents. As you progress through the mode, you will unlock new characters by defeating them in battle.
Play through Spirit mode: Spirit mode is a single-player mode in which you collect and train spirits, which are characters from a variety of franchises that can provide special abilities and bonuses in battle. As you progress through the mode, you will unlock new spirits, which can include characters from Super Smash Bros. Ultimate.
Play online matches: As you play online matches, you will occasionally be challenged by a "Master Hand" character, which is a special boss character that can be unlocked by defeating it in battle.
Overall, there are many different ways to unlock new characters in Super Smash Bros. Ultimate, so keep playing and experimenting with different modes and characters to unlock them all!