Brokerage investments
Contact the custodian with death certificate and transfer-on-death or estate paperwork as applicable
Some steps differ by region inside the country.
Where
Each brokerage or investment platform, and their financial advisor if they used one.
How
Look for statements and tax documents in their mail and email to identify every firm.
Ask their advisor for a consolidated statement of everything they managed.
Ask each firm whether the account has a transfer on death registration, since that bypasses probate.
Request the date of death valuation, which the heirs need to establish their stepped up cost basis.
What to bring
A certified copy of the death certificate
Account numbers
Letters testamentary, or the transfer on death registration details
Each beneficiary's account details for the transfer
This is practical information, not legal, medical, or financial advice.