WHOLE CHARACTER
天tiān
sky; heaven; day (今天, 明天); weather (天气); natural; innate (天性, 天才)
天 is the sky — and a day, the weather, and what is heaven-given.
The header calls the formation 指事 — a mark (一) above 大 for what is above, the sky; shown here as two pieces.
TOP MARK (THE SKY ABOVE)
一yī
one
Structural source piece in 百; not every visible stroke-like piece is taught as a separate word.
In 天: top mark (the sky above) Shown with its documented or clearly limited role.
THE LARGE BASE FIGURE
大dà
big; large
A standalone character; kept separate from the unclaimed visual role in 参 and the base-figure role in 天.
In 天: the large base figure Shown with its documented or clearly limited role.