40 is no more significant than other numbers, and considerably less so than numbers such as π or e.
The significant figure of 37.753 to 1 significant figure is 40.
Rounded to one significant figure it becomes 40
40
40
4
40
10080 has 5 significant figures - so 1 significant figure would be: 10000.
To one significant digit, 37.19 is 4x101. Writing it as 40 makes it unclear as to whether you have 1 significant digit or 2.
For doing significant figure you down count from the start the required amount, so 645533 would become 646000.
Left zero's are NOT significant: So it's one in 0.01
In 10.00, the first significant figure is the "1", and the last significant figure is the last "0", because zeroes to the right of the decimal are significant. The two central zeroes are also significant because they are between significant figures, so 10.00 has 4 significant figures.
Truncated to one significant figure, it's 9,000 .Rounded to one significant figure, it's 10,000.