You have it backwards - radioactive decay turn mass into energy. (that's literally what E=mC^(2) means). The conversion multipler of mass to energy is C^(2) so a huge amount of energy makes a teeensy tiiny bit of mass, or a tiny bit of mass makes a HUGE amount energy.

Fusion in stars is complicated, is uses not-ELI5 type things like the "weak nuclear force", and yes, gravity, to convert energy into mass and energy into different mass and energy. The trick in the Sun is it has a tremendous amount of Gravity which kick-starts the process and creates a large net-output of energy (sunlight and other radiation). In something a fusion powerplant the problem is you don't have that gravity source so you need a large energy input in the form of heat in the place of gravity. Currently, it takes more energy for us to create fusion that we get out so it's not a sustaining reaction like the Sun is.