Yeah I agree this is for the wip section. but to answer your question, proper eqing is a huuuuge part of getting a smooth sound. along with subtle uses of chorus, phasers, and reverb. also if you sidechain a delay/reverb to your lead, it can make it more interesting and sound smoother
edit: btw I didn't listen to the example because I'm in school atm, those are just some general tips that I thought would help