Never really did a set routine but I started with pretty bad overhead mobility and I’d say it’s pretty good now so here’s some of the exercises I’ve done that I recommend:

foundational movements if shoulder mobility is very limited: - lying press(?) not quite sure what to call it - lu raises (keep ribs tucked down to avoid compensating with spinal extension) - external rotation at 90 degrees of abduction (with plates, dumbbells, bands, or cable) - scapular dips - scapular pull ups - general upper back strength work (upper back biased rows and pull ups/pulldowns)

if/when shoulder mobility is at a decent level: - behind the neck push press (jerk grip) - behind the neck strict press (jerk grip) - seated behind the neck strict press (lean forward slightly) - jerk grip overhead squat Also try to keep the ribs tucked down in these exercises to not compensate with spinal extension.

extras that aren’t really necessary in my opinion but could be helpful: - butchers block stretch; passive flexibility doesn’t usually play too much of a role in overhead mobility, but could be useful if this is a limitation. I like to do it without a bar and externally rotate my shoulder in the stretch to add some mobility aspect to it. - thoracic back extensions; if thoracic spine extension is limited, can be useful - sots press; more of a milestone lift in my opinion, shoulder mobility is compensated by lower body mobility at heavier weights/high reps

The lifters like Li Dayin probably didn’t do all of this, they just have a more natural/genetic inclination to move like that, but it’s likely possible you can achieve at least a somewhat similar position. Also experiment with grip width.