The question you opened
How full is the sweet jar
One handful holds 8 sweets, and the jar looks about 20 handfuls full.
Roughly how many sweets are in the jar?
about 160 sweets
One handful times the number of handfuls: 8 times 20 is 160. The handful is the measuring cup for the whole jar.
A handful holds 8 sweets, and the jar looks about 20 handfuls full. So the jar holds roughly 160 sweets. The arithmetic is one step: 8 times 20 is 160. What makes this an estimating question is everything that happens before the multiplication. Nobody can count the sweets in a jar by looking at it. What you can do is count something small and easy, then work out how many of those small things fit. The handful becomes a measuring cup.
Count once, carefully, and the rest is multiplication. That is the whole recipe, and it works on almost anything too big to count. How many blades of grass on a lawn: count a square the size of your hand, then work out how many hands fit. How many words in a book: count the words on one line, multiply by lines on a page, multiply by pages. How many bricks in a wall: count one row, then count the rows.
The answer will not be exact and does not need to be. If the handful was really 7 sweets, the jar holds 140. If it was 9, the jar holds 180. So a sensible answer is somewhere near 160, and anything between about 130 and 190 is a good guess. Estimating is about landing in the right neighbourhood. A quick way to check yourself is to ask whether the answer is silly. Sixteen sweets would be almost an empty jar.
Sixteen thousand would need a jar the size of a bathtub. One hundred and sixty feels like a jar you could carry, so the number passes. One more habit that helps. Round your numbers before multiplying, not after. Eight times twenty is easy in your head. Seven times nineteen is not, and it gives you an answer no more truthful than the easy one, because the count of handfuls was a guess in the first place.
Technique: Count a small part, then scale up