In Fuser, the player takes the role of a DJ at a large concert and make a mashup of different songs. They have a virtual DJ table with four record players, custom instruments, sound filters, and up to 24 records (their crate) along the top of the screen. Each record has four tracks keyed by color and related to a type of musical instrument such as drums, guitar or vocals. At any time, the player can place a record, then select a specific track from that record, and then place it on any of the record players. This adds that instrument's track to the current mix across all four records, automatically adjusting the mix's tempo and musical key. New records/tracks can be added to any record player, overriding the previous track in the mix. Additionally, each player can be cued up with a second instrument track, which can be then switched back and forth individually, or all four players can be changed to the new ones by a single drop or using a "riser" that transitions them over a short period. Audio filters can be used on each instrument, and the user has the ability to create various instrument loops via a multibutton synthesizer which then can be added as another instrument to the current mix. This concept is comparable to Harmonix' prior game DropMix, which was a physical card game incorporating NFC technology with an electronic game board and mobile app that was published through Hasbro.
The game has a single-player campaign featuring multiple sets across various venues. At each set, the player attempts to make mixes, scoring more points by adding new tracks either on the downbeat or at specific beats by each instrument where the track drops. Additionally, the virtual audience will throw out requests, such as for specific songs, songs from a specific genre, or a certain type of instrument. Completing these requests in a short amount of time scores additional points. The player is ranked on a five-star scale based on their total score during the set. To achieve a high score, the player has the opportunity to customize the contents of the crate to be as efficient as possible in switching between tracks.
Fuser also includes both cooperative and competitive multiplayer modes that let players collaborate on creating a mix, or battle against each other. Players can also participate in weekly mix events that tasks players with creating a mix based around a particular musical genre or instrument that is then voted on by other players. The game also features a freestyle mode that allows players to create their own mixes and share them with others over social media.
Harmonix introduced the Headliner Spotlight feature in May 2021. This presents a few "Diamond Stage" dedicated Twitch channel which players can perform Fuser mixes in front of viewers. Players can earn in-game diamonds from other game activities to reserve spots on the Diamond Stage as well as to unlock additional cosmetics, songs, and other features. While a player is performing on the stage, viewers can submit requests, fashioned similar to those in the game's campaign mode, to challenge the current player.