As we know that US government announced $2T Covid-19 stimulus package on March 24. It includes a payout of $1200 to all taxpayers.

Who are the individuals who will get that benefit? Is it all US Taxpayers or only US Citizens? Will a resident alien on work visa, who pays taxes, get that benefit as well?

No. The benefit is issued as a tax credit by amending the Internal Revenue Code of 1986. It applies to all US taxpayers subject to the definition of "Eligible Individual" in the CARES Act, which is as follows:

“(3) ELIGIBLE INDIVIDUAL.—The term ‘eligible individual’ means any individual other than—

“(A) any nonresident alien individual,

“(B) any individual with respect to whom a deduction under section 151 is allowable to another taxpayer for a taxable year beginning in the calendar year in which the individual’s taxable year begins, and

“(C) an estate or trust.

To explain subsection (B) in layman's terms - if you are listed on someone else's tax return as a dependent, you are ineligible for the check.

So as long as the alien is resident (i.e. has a green card or passes the substantial presence test), not someone's dependent, and assuming they pay enough tax and earn less than $99,000, they will be eligible to receive the check.

Note the stated amount of $99,000 comes from the full check value of $1200 being reduced by $5 for every $100 you earn over $75,000, therefore falling to $0 after $99,000.