Is the bottom of a box does not have a part that tuck in to seal the box still called a flap?

Is the bottom of a box which does not have a part that tucks in to seal the box still called a flap?

No, the bottom of this box is not a flap. It would be referred to as the “bottom”, “base” or “underside” of the box.

Consider this definition of a flap from dictionary.com

something flat and broad that is attached at one side only and hangs loosely or covers an opening:
the flap of an envelope; the flap of a pocket.

next,

apparently the top (labeled blue) of the box shown below is a flap.

There could be different ways to describe this.

1. Technically, yes. The entire piece labeled blue is a flap.
2. It could also just be referred to as "the top of the box" since it's a single large section which covers the entire top.
3. You might choose to call the smaller folded end section of the top "the flap", as in "not the whole thing, just the flap part." Not sure if this is "officially correct" but a non-box expert would probably understand what you mean.