If you mailed the federal amended tax return by certified mail through the US Post Office, they would have given you a return receipt when it was accepted by the IRS.

The IRS was not processing any type of paper tax return for several months due to the pandemic and limited staffing.  They have only just recently started to process paper returns based on when they were received.

Go to this IRS website for the status of an amended tax return - https://www.irs.gov/filing/wheres-my-amended-return

Expect amended returns to take 6 months or more to process this year ... once your envelope is opened and your return is in the system then you can track it here :  https://www.irs.gov/filing/wheres-my-amended-return

If you requested a return receipt, even in the best of times (in my anecdotal experience) the IRS does not always comply with the "return receipt requested" since they get so much mail in bulk.   With their several-months stockpiling of mailed returns due to COVID, the situation is compounded.   Tracking on the USPS website might be more accurate, but there's no guarantee the final delivery will show up there, either, since in a bulk dropoff the USPS may not have scanned it on arrival in the absence of an IRS recipient.   So you're sort of in limbo as Critter-3 mentioned above.