Let me simplify it further for you.
For example:
Mr A sent you 1BTC
Mr B sent you 2BTC
Mr C sent you 3BTC
1BTC + 2BTC +3BTC = 6BTC
That means you can have 3 inputs if you want to make transaction (Mr A, Mr B and Mr C coins sent to your address) of 6 BTC of UTXO
Assuming you want to transfer the whole 6 BTC to MR D, that means you are spending the whole 6 BTC of UTXO as 3 inputs as you received the 6 BTC in 3 different transactions.
If Mr A, B and C sent the coins to different addresses generated on your wallet, that means the 3 addresses can be know on the blockchain after you spent the 6 BTC, they will be seen as inputs that belong to addresses generated on the same wallet with seed phrase, seed or master private key.
You are though not wrong if only the wallet has 1 input of 5 BTC, it means the bitcoin wallet only receives 1 input (like only Mr A sent you 5 BTC) to spend from. If you transfer 3 BTC, then 2 BTC will be sent to your wallet change address.
To maximize your privacy, know that wallets that automatically select UTXO as the input can select it in a way many addresses can be connected together, you can use coin control on wallets like Electrum to manually select the certain input of your choice. You can even manually select the change address of your choice.
But what should more noticed is that most bitcoin wallet have no coin control and they select inputs in a way many addresses can be linked together.
They can conclude it, if say they're the merchant. But, everyone else who's watching the chain can't prove it's yours.
Exactly. But the worst aspect of this is when many addresses are linked together as inputs which can be seen on blockchain explorer. The output can be a guess but the input is not a guess, it is definite.