Short answers, pulled from the story.
The decimal system is based on 10 most likely because humans have ten fingers, and counting on fingers is the most natural starting point for a number system. Ancient cultures from the Indus Valley Civilisation to Egypt independently arrived at base-ten systems, reinforcing this anatomical explanation.
The Persian mathematician Jamshid al-Kashi provided the first systematic and comprehensive treatment of decimal fractions as a complete system in the 15th century, in his work The Key to Arithmetic (Miftah al-Hisab), predating comparable European developments by nearly 175 years. Earlier contributions came from the Arab mathematician Abu'l-Hasan al-Uqlidisi in the 10th century and the Chinese mathematical tradition dating to the 3rd-5th century CE.
John Napier introduced the period to separate the integer part of a decimal number from the fractional part in his book on constructing logarithm tables, published posthumously in 1620. Simon Stevin had earlier introduced a forerunner of modern decimal notation in his 1585 Dutch booklet De Thiende.
A terminating decimal is one where all remaining digits eventually become zero and can be dropped, such as 0.25. A repeating decimal is an infinite decimal where the same sequence of digits repeats indefinitely, such as 0.333... A number is rational if and only if its decimal expansion either terminates or eventually repeats.
The world's earliest positional decimal system was the Chinese rod calculus. Positional systems assign value based on a digit's position, allowing the same symbol to represent different quantities depending on where it appears in a numeral.
No. Several cultures use or used different bases. The Maya used base 20, the Yuki language of California uses base 8, the Huli language of Papua New Guinea uses base 15, and the Ndom language of Papua New Guinea uses base 6. Early Germanic languages also show evidence of a long-hundred system based on 120 rather than 100.