i'll try to explain.
this gogg is one of first hguc, (uc = universal century) a step up the old hg, this is one of the more simply between this series , have not inner details as mg or pg (the skeleton inside like the blue MG gouf i show before) . Actually not represent the hguc standard, it was one of first attempt to improve old hg.
all joints in this series are covered ,resulting more realistic but, as you can see lots of works needed to make it more realistic. On latest exit of this series have some kit that instead have some great more details like the giant zock of my actual avatar, acguy and some other have great enhanced posability.
the hguc zock (one of the largest hguc) with all under parts detailed visible
and the acguy too have under details, only the body is without
Pay attention that i added panel lines on both kits , as original they comes without it.
and here a confrontation with the same subject, rx-79g, one like your hg and the other the new hguc,
even if the picture is blurry you can see that as example , the feet, the older kit resolve with 3 pieces and the
joint in soft plastic, the newest have 6 pieces and the soft joint and the toe is movable , plus the up ankle armor is movable , and in the older are moulded with the ankle.
the elbow on older have an only rotation point , the new have two point, better hands , better weapons etc
hope this usefull and not mess you 
To add on series to see youcan try even the 0080 pocket war; on there have some crude scenes (are not walt disney cartoons), but look at the war with the eyes of a child that as fan of mechs , see suddenly involved his city in a battle between giants, that causes even civil casualties, there have (imho) most of beautyfull mechs never seen on a show 