federal estate taxation

I am trying to understand how this works specifically, if the rules are cut back to the 2009 levels where the exemption is 3,5 million ( or worse if Biden has his way).  Let's suppose hypothetically, a married couple has an estate valued at 6 million, which consists of 5.5 million in an IRA and 500,000 house value.  Now, suppose one spouse dies. As I understand it, there is no estate tax considerations because the surviving spouse will inherit the IRA and house tax -free ( assuming beneficiary designation has been set in the IRA).  First question : are there any special tax forms that would have to be filed due to the first spouse death, other than the regular 1040 ?  Now suppose a couple years later the second spouse dies with the 6 million dollar estate and suppose the federal exemption has been reduced to 3.5 million.   This would mean that the excess over 3.5 will be subject to estate tax which would rapidly grade up to almost 50%.  Here is where I want to understand what the process is  for the executor to follow.  What are the specific tax forms that he/she would have to fill out? Would a 1040 have to be filed for the deceased spouse?  Thank you