That's franky's thing to bring up, and it's kinda pointless to even discuss if you pay an on-chain transaction fee that's a million times larger.
Yes, but it is not about the amount, but about the ownership: if you want to have millisatoshis on-chain, then in the current version, you can only have a large multisig, behind a single satoshi, so if you have 500 millisatoshis from Alice, and the same from Bob, then all you can do, is to use 2-of-2 multisig on a single satoshi.
How are the Bitcoins in those payment channels in Lightning "IOUs"?
I think they are not. But if people say that coins in sidechains are, then coins in LN should also be called in the same way, if we follow the same logic.
Where/how are the coins stored/locked and how are the "IOUs" issued?
You have IOUs only in centralized sidechains. For example: if you have a sidechain, where you have some large multisig, and you cannot be a miner, if you are not a part of some closed group, then those coins are IOUs. However, if you have decentralized sidechain, then you can use any scripts you want, because every block is covered only by hashrate, and not by some centrally-selected signature, like in signet.
But, I'm confused, kindly explain to me how a Bitcoin-pegged-coin in a sidechain is an actual Bitcoin.
1. If you have to provide a valid signature for a given coin, to transfer it from mainchain into sidechain, then it is based on real coins, and not some "coins created out of thin air".
2. If you have to move a given coin on the mainchain, to bring it back from the sidechain to the mainchain, then it is the same situation as with LN: you can always peg-out, without asking for anyone's permission.
And those two things alone: sign things to peg them in, and move them to peg them out, is sufficient to make a test network for decentralized sidechains. However, if you want to get the main network, then it is all about creating rules for the scripts, used to lock your coins. Which means, that if you have a single-key address, then you can test it alone, but no sane person will accept some coins, if the sender can always take them back, without losing anything. For that reason, the basic building block for decentralized sidechains is N-of-N multisig, where you can have a lot of people behind a single UTXO. And then, all kinds of improvements are focused on making "emergency scripts", to for example not require all N participants to be online, to change the state of the sidechain.
Because if you would have for example non-interactive transaction joining, then a single Taproot UTXO is all you need, to handle a single decentralized sidechain. And then, it is all about combining all signatures from that sidechain, and finalizing it as a simple commitment on-chain, which would have a constant size, regardless of how many people will use a given sidechain.
What is the process of on-ramping Bitcoin in a sidechain, and what is the process of on-ramping Bitcoin in the Lightning Network.
It didn't change that much since the last time, when I wrote about it:
https://bitcointalksearch.org/topic/m.61778369