Playing Mario Games Online: Official & Community Options

Official Nintendo (Best Experience)

• Nintendo Switch Online ($20/yr): NES/SNES/N64/GBA apps — Super Mario Bros. 3, World, 64, Advance 4 + online play
• Super Mario Maker 2: Create/share/play millions of levels — online co-op/versus
• Mario Kart 8 Deluxe: 48 tracks, 200cc, online races/battles
• Mario Party Superstars: Board game online, minigames
• Dr. Mario World / Mario Kart Tour (mobile): Free, online, microtransactions

Community / Emulation (Gray Area)

• Netplay (RetroArch / Dolphin / melonDS): Host/join rooms for NES/SNES/N64/DS Mario — requires owned ROMs, config effort
• Kaizo / ROM hack communities: Discord servers, SMW Central — custom levels, races
• Mario Royale (fan, DMCA'd) / DMCA-free clones: Battle royale Mario — browser-based

Legal & Safety

• Emulation legal; downloading ROMs you don't own = piracy
• Nintendo DMCA's fan servers — use at own risk
• Official NSO = supported, stable, legal, cross-play with friends

Recommendation

Start with NSO + Mario Maker 2 + MK8D on Switch. Covers 2D, 3D, creation, racing, party. $60 console + $20/yr = years of Mario online.