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Road Fan, in the few modern freewheels I can service, internally there is basically no difference mechanically from 1980s Shimano, Sachs, Suntour Winner Pro, and Regina freewheels.
Now with that said, I need to admit failure in opening up any Sunrace or IRD freewheels. The retaining ring/outer bearing race will either not budge or the smallest sprocket cannot be removed, thus blocking access to the retaining ring. In some cases a special tool is needed, and I've not been able to locate a source for the tool and have not invested in the cost to have one made.
The weight reduction you notice is in the thickness of the sprockets (as mentioned by @
JohnDThompson) and also the amount of material removed from between the body opening and the teeth on the sprocket. Older sprocket manufacturers seemed to be more conservative with the amount of material which was removed, IMO.
For this reason I've been able to warp while riding a few Shimano Mega Range largest sprockets on 6 and 7 speed freewheels.
