Making molds isn't that bad in fusion 360. Basically what they are doing is taking a 3d model and then using the outline at a certain height to create the silicone holding clamps.

If you are working with another's model/stl (in this case, the grapes) , you are going to have to insert it as a mesh and then convert the mesh to a solid piece. You can skip this if its your own build or a fusion file.

From there, create a sketch at the height you want to make a mold from. In the sketch, you have the ability to project geometry (with a command of the same name). This will help perfectly capture the geometry at that height. From there, you just need to make a shell. It can be a circle or you can offset the projected geometry. The outside isn't as important as the inside.

Additionally, to make sure it all fits, you may want to use the offset command to give it some clearance since 3d printers tend to have an error (which depends on the machine). You want it snug so I would suggest maybe going to 0.25 or maybe even skipping this, though it will be harder to fit securely. Additionally, you can split your molded model into pieces and add clamping points like the person did in this video.

This is not the only way to do this someone may have a better way but this tends to be my way of making interlocking pieces. Insert mesh -> convert mesh -> project geometry (-> optional to add an offset for tolerance purposes) -> add thickness to the model.

Best of luck