There are several DLC packs available for Mario Kart 8 Deluxe, each offering new content, including:

The is a massive expansion for the best-selling Nintendo Switch title, adding 48 remastered tracks and 8 characters to the base game.

DLC and technical considerations regarding files for users of the Nintendo Switch. 1. Overview of the Booster Course Pass DLC

While the term is technically accurate (the DLC is originally packaged as an NSP by Nintendo), searching for and downloading these files from the internet is almost exclusively related to console modification and piracy.