WHOLE CHARACTER
合hé, gě
to close; to join; suitable; to fit; to conform; altogether
合 closes and joins — a lid over a box: 合上 the book, suitable (合适), working together (合作), up to standard (合格).
ZDIC gives 上中下结构 and 会意 without naming pieces; the three written layers are shown as visual pieces (a lid over a joined box).
TOP: THE LID PIECE (VISUAL)
人rén
person; people
Repeated in 众.
In 合: top: the lid piece (visual) A written piece here; not taught as the word rén 'person'.
MIDDLE: THE JOINING LINE (VISUAL)
一yī
one
Structural source piece in 百; not every visible stroke-like piece is taught as a separate word.
In 合: middle: the joining line (visual) Shown with its documented or clearly limited role.
BOTTOM: THE BOX (VISUAL)
口kǒu
mouth; opening
Mouth/speech/opening clue; may also be a repeated visual element.
In 合: bottom: the box (visual) Shown with its documented or clearly limited role.