You may want tax and legal help this year. Some general rules and comments.
1. If he has taxable income (pension, etc.) then someone needs to file a 2023 tax return for him, that will be the person he chose as executor of his estate or the person named by the court to handle his affairs. His income is not combined with anyone else's income. He would file the same kind of tax return as if he was alive, except that it is signed by his representative.
https://www.irs.gov/individuals/file-the-final-income-tax-returns-of-a-deceased-person
2. When you inherit property, you receive a "stepped up basis." That means that your cost basis in the property is equal to the fair market value on the date he died. With respect to a house, that means you probably did not have any taxable gain when you sold it, assuming you sold it for about what the value was on the day he died. If you received a 1099-S at the sale, you need to report it, but it is not taxable income. If you donate personal belonging to charity or sell them, you can generally take a tax deduction for the fair market value of the property as of the day he died, and you won't have taxable income as long as you sold the items for their FMV on the day he died. (I am assuming that most items would not radically increase in value in a few weeks or months.)
3. Money received after your grandfather died goes into his estate. This is a legal entity that only comes into existence when a person dies. If the estate has substantial income, then the estate may need to file a special tax return before that income can be passed on to his heirs. But you generally won't be dealing with an estate unless he has some kind of ongoing business activity (like book royalties, etc.)
4. Money you inherit is not taxable. But income paid to you may be taxable. For example, whatever pension he received before he died was taxable to him and would be reported on his final tax return. But if additional pension payments are received by you, they will be taxable income to you (because they would have been taxable to him) even though the money you get from cleaning out his checking account is not taxable.
