Zio Mc on Nostr: I've got a question about #bitcoin transactions, how can be sure no one can modify ...

I've got a question about #bitcoin transactions, how can be sure no one can modify the utxo i write in transaction when is in mempool?
I'm reading a lot of documentation but i cannot find whether utxo fields are signed with my private key.
In case utxo is signed i cannot understand how coinjoin can work trustless.