Time.

Those who don't enjoy the gift of natural rhythm or an inherent tactility which serves to play alternate patterns, or with varying attack and tonal emphasis semi-subliminally just have to work consciously at it until it becomes so. Pretty much the same as all things guitar really. Be tenacious, put the work in, achieve the result.

I found what you're referring to relatively easy, some even instinctive. But easy is qualitatively relative. Maybe what I consider light going you consider hard work? Even so, I still have to put the work in to get it right with unusual if strumming, or complex patterns.

Watch the tute paying attention, focus on the detail and nuance required to achieve your objective, slowing it right down, play it aiming for accuracy over and over until you can consistently and accurately. Now gradually increase the tempo in stages until you can play it just like that at exercise or song tempo. If the riff requires altered patterns etc, break it down to the individual chords or sections and practise each as until you are competent with it , then stitch it all together as the whole.

Ultimately, there's no magic wand or wizard which will substitute for time on the instrument. Broken down to the simplest equation, although it's a cliche, the more I play, the better I get. You will too.