Singapore had only a few Malay fishermen as inhabitants at the time of its founding as a British trading post in 1819. It was subsequently, and quite rapidly, populated by immigrant peoples, primarily Chinese but also Malays (from Sumatra as well as adjacent Malaya) and Indians (who took advantage of common British governance to migrate to Singapore in search of better employment). Thus immigration, rather than natural increase, was the major factor in Singapore's fast population growth through the mid-20th century.
In November 1965, following separation from Malaysia, Singapore's newly independent government introduced measures to restrict the flow of Malaysians entering the country in search of work. These immigrants, who averaged 10,000 a year up to 1964, had to establish residence for several years to qualify for citizenship. In addition, all non-citizens were required to apply for a work permit or employment pass. Immigration is now generally restricted to those with capital or with special skills. In 2000, the net migration rate was 19.6 migrants per 1,000 population. There were 1,352,000 migrants living in Singapore in that year.
I believe you are a Nepali student,though you didn't find it important to mention.
And you are a Migration-Geography Student (in NUS), and this answers all your questions. A NUS student (from Nepal) had such a poor English and poor selection of words and poor grammar. What would you expect from general workers?
Are Nepalese helping improve Malaysian economy or their own? (I guess many more Nepalese are jobless in Nepal than the workers Malaysia needs)
And YOU tried to find "I have not seen any Nepali is over there and I tried to find that why Nepalese workers are not in such sector in Singapore." Very good research. Where did you (try to) find? What did you find? Whom did you ask, the construction workers? I guess that was your best choice. To know about government policies, you ask the blue collar workers.