How does 巨 resemble a hand (彐) holding a carpenter's square (工)? Left screenshot is from Yellowbridge; right from Richard Sears (Uncle Hanzi). 



I circled in green the lone pictogram that most obviously pictures a hand holding a carpenter's square.


For productivity, I colored in red a semicircle in merely 3 of the pictograms, but this semicircle figures in the other pictogram. Are these semicircles intended to symbolize the human hand?
I still can't picture how these pictograms evolved to 巨.
The forms you're looking at are actually the ancestors of「矩」.「巨」was formed by cutting off the person.



The analysis starts from this form

西周金伯矩盉蓋集成9412

which depicts a person「大」holding a tool「工」.


  Carpenter's square is much too specific, and I would think that it is an over-interpretation.



Exaggerate the person's hand to emphasise that the tool was held:

西周金伯矩卣集成5228
Swap out「大」with the similar-meaning「夫」(man), and permanently detach part of the hand onto the tool:

西周金裘衛盉集成9456


The left hand side of the above character should now heavily resemble「巨」, which is created from omitting「大」or「夫」.

秦簡睡・語5　篆工部說文解字今楷　


  「夫」was later semantically corrupted into「矢」(arrow > length, measurement), forming「矩」.
  
  東漢隸桐柏廟碑　今楷　




References:


季旭昇《說文新證》
小學堂
國學大師


中國社會科學院考古研究所《殷周金文集成》