If both the deceased and beneficiary are not US resident, what is the procedure of claiming inheritance(the deceased has possibly held brokerage account in US)?
Normally, a U.S. brokerage firm would honor a court order from the jurisdiction where the decedent was domiciled at death concerning succession to ownership of the brokerage account if:
There was no beneficiary or joint owner of the account.
There is adequate proof that the account holder is dead in the form of a death certificate.
There is adequate proof that the decedent was domiciled in the place where the court order was issued (such as an affidavit or finding of fact in a court order).
There is adequate authentication, and if necessary, translation, of the court order, for example, with a certified copy of the court order, an apostile indicating that the person certified it could do so, and a sworn affidavit supporting the translation's accuracy and the translator's qualifications.
If the brokerage firm did not cooperate, the executor of the decedent's estate could open up what is called an "ancillary probate" in the court with probate jurisdiction in the place where the brokerage firm was located, which would cause the executor to also be appointed by the local court, and the brokerage firm would then honor the executor's directions.
Often, the best first step is to contact the brokerage firm, advise them of the death, and ask them what process it would need to be satisfied that it can transfer the account. Typically, each firm has its own checklist and forms to complete.
If the account is to be treated as part of the general estate, and go through the probate process, the executor will handle the matter. Probate would probably be based in whatever jurisdiction the deceased was domiciled in.
If the matter is to be handled as a direct transfer outside of probate, the brokerage will have its own procedures. The best way to find out about those will be to write or call the brokerage. They will probably need proof of identity. There may be tax implications, depending on the laws of the jurisdiction where deceased was domiciled, or where the beneficiary now is. The brokerage may handle that, or some of it, but the beneficiary would be wise to consult a tax expert or lawyer if the sum is sizable.