Why's flesh (⺼ ) the semantic component for 肩, 脖? I know that 脖 (neck) and 肩 (shoulder) contains flesh, but they aren't the fleshiest, most sarcous or adipose, unlike the butt.

If CUHK is proclaiming that 肩 resembles a shoulder, I can't see the resemblance.

I also don't understand how the shoulder can be &quot;the body's gate&quot;. Yellowbridge




CUHK and Yellowbridge don't expound 脖.
If CUHK is proclaiming that 肩 resembles a shoulder, I can't see the resemblance.

商甲乙6638合集14199商甲後2.17.13合集13917商甲鐵215.4合集4786商甲甲759合集35191
The top left of 「肩」 was originally a picture of the shoulder bone (scapula), most easily illustrated with those used in oracle bone divination from oxen:


(Left) Oracle bone from the Shanghai Museum*. (Right) Binary image of this oracle bone given in 《甲骨文合集》32028.
*Credits to Herr Klugbeisser, CC BY-SA 3.0, original photo at https://en.wikipedia.org/wiki/Oracle_bone#/media/File:Orakelknochen.JPG.

Later on, the character became more stylised and abstract. 「肉」 (flesh &gt; body part) was added, perhaps to enhance character recognition of what might otherwise highly resemble 「戶」 or 「耳」.
戰國・楚簡乙4.61新蔡葛陵楚墓竹簡秦簡日甲75背睡虎地秦簡隸定　
The bone-shape was later firmly corrupted into 「戶」.
東漢隸武梁祠畫象　楷　

References:

季旭昇《說文新證》
小學堂
國學大師

郭沫若《甲骨文合集》
中國社會科學院考古研究所《殷周金文集成》