I have 2 BIG questions about Bitcoin transactions.

I'm trying to understand some things about Bitcoin, and I've been searching for 2 days, watching videos, reading articles... None of them have explained it to my absolute understanding. It's frustrating.

Question 1 (Transaction Inputs):

Where EXACTLY do the inputs come from? I learnt they are UTXO's (Unspent Transaction Outputs) from previous transactions. Which transactions, and to whom? Does every payment that comes to me generate a new UTXO? So for example, if 3 people have recently sent me BTC (to a single address I own), do I have 3 separate UTXO's to spend (use as inputs for my next transaction)?

That was my reasoning, and it seemed to be making sense, until I came across info that said the inputs can come from different addresses. Huh? Okay... Looks like they have a point though, because when I check the block explorer for transactions, I actually see distinct ADDRESSES as inputs and outputs. It should mean that every address can rather only have one UTXO at a time, right? And multiple UTXO's should definitely come from different addresses. But if multiple addresses are used as inputs, shouldn't all of them be owned by the sender? Because only the sender should authorise a transaction.

Now if that's the case, how is that done? Because with the wallets I've used (Trust and Binance), I don't see an option to select multiple addresses I own and combine their value when I'm sending BTC. So... What's happening? Can multiple inputs only be achieved using low level coding or something? When I search for "How to send Bitcoin from multiple addresses" or similar, I don't find anything that answers my question. It's as if that's not possible. I'm just not getting it. For the outputs, I understand. But the inputs haven't really been explained to me.

Can someone who understands this from A to Z spell it out for me without leaving anything out? I'd be really grateful.

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Question 2 (Block confirmations and reversibility):

Many sources say that transactions that have 6 or more confirmations can be considered "irreversible". Okay, logically, that should mean that with less than 6 confirmations, the transaction can be reversed.

Now, what I know about block confirmations is that it's the number of blocks that have been added to the blockchain after the one that contains the transaction in question. So that means even with one or two confirmations, the block is actually on the public blockchain, right? So if the transaction can be reversed, how is that done? Will it be pulled down from the blockchain? (funny question). What would happen to the 1-5 blocks after it? Meanwhile if you search for "Can bitcoin transactions be reversed?", all the answers you get are "No." So what's really the point here?