While level editors are hardly a new phenomenon, the release of Mario Maker for Wii U in 2015 did seem to trigger a wave of fan-built editors. We've seen a classic Zelda Maker (RIP), a Mega Man Maker, and now, a level editor for Super Mario 64.
It's not as straightforward as the others: it's a ROM hack, which means you need an emulator and a ROM for Mario 64. It also uses the classic Nintendo 64 controller, which will no doubt present a problem for many.
But if you've got those, the video above is a good indication of how it works: basically choose environmental elements, plonk them where you want, and then jump in and play. It's the work of Kaze Emanuar, who has worked on Mario-related ROM hacks before. Whether this one can survive Nintendo's rather inconsistent approach to issuing takedowns to fan-games and hacks, is yet to be seen.
Instructions for how to obtain and install the hack is in the YouTube description. I haven't tested it myself, so you do so at your own risk.