I am currently a student (above 21yo). I have a few debit cards used for shopping and daily needs. I dont have any cashflow issues or budget problems, and I do not spend unnecessarily. Should I apply for a credit card to improve my credit score?

As you grow older, it will make more sense to have credit card as you will be able to get more rewards, however at this point of time I don think there is a need for you to get a credit card as you are still a student afterall.

Yes you should maximise the use of credit cards to earn cashback/miles on the spending that you would ALREADY have been spending. But do be careful on those CC with min spend per month in order to hit a certain percentage or cashback etc. If you're not careful, you may be spending more than you can afford.

Building up your credit score using credit cards is always good as banks will be more willing to lend you (in terms of loans and future cc application) if you display good behaviour such as paying on time and in full).

No, avoid credit cards (exception: travelling abroad, or if really needed for online buying)

if possible

Do apply for at least one as one factor for credit score is maturity - having a history of manging credit well is positive (whereas new credit files tend to have a tag "score is impacted by relatively short credit history")

as a bonus; the bank will likely access your credit history and that will by law allow you the chance to get a free credit report to understand further what factors are involved in the scoring.

of course all of this relies on being disciplined about your spend on credit card and repayment - micro managing the above is not helpful if you don't repay on time :)

(but keep disciplined about using it;