Medium jelly is a stronger version of the light jelly healing item. That said, to get medium jelly in Scarlet Nexus, you will need to do either one of two things, the first is to find it while exploring and the second is to exchange materials for it.
If you want to go for the exploring route, we recommend checking the abandoned subway you first get introduced to in chapter two or phase two. In this location, we found that the jelly has a slight chance to drop at one of the various pickable resource points, so because of that, it couldn't hurt to check the subway.
But if you rather just exchange materials for medium jelly, you can do so at Satori the Archivist's shop - you can also just buy it at the shop once you reach a certain point in the game.
That being said, once you have some jelly, you will be able to use it to heal you more than the light jelly in combat. Along with this, you will also need two medium jellies to complete the Because it's Good quest.
However, if you haven't got this quest yet, you do so by speaking to an OSF soldier near in Souh City Ryuhjin Ward - just look on the min map for a diamond shape to find them.
Check out all of our guides for Scarlet Nexus!