OP, it is nice that you have come up with this article. I am not a tech guru, but at least I have been in the space for a little while now, and what I know most is that the higher the amount one puts as a transaction fee, the faster the confirmation, but sometimes it seems there is no competition on the mining block rewards (please spare my choice of words), as one can set a little transaction fee and it is quickly confirmed.
This is because at those times the mempool is less congested and the depth of the mempool is very shallow, so the acceptable fee rate goes low since there's not many transactions for miners to be selecting only those with higher fees. They include almost every transaction into their block and get it confirmed, even those with fees lower than acceptable fee rate.
Lastly, I had an experience with the Trust Wallet app when I tried to transfer some Bitcoin to another wallet, but whenever I tried to do such, I always got an error message as UTXO ERROR, even when I increased the fee, the same response I got, and I did this repeatedly, thinking it was a network error. I checked for a wallet update, and the wallet version is up to date. I had to download another wallet and import the private key into the wallet, and I did my transaction without adding any fees. This made me wonder what really happened because I thought that if I increased the fee, the transaction would scale up, but it did not. Can you explain what really happened there?
I would wait with you to get the reply on this because I've never heard of a Bitcoin transaction that got confirmed without fees.