Multi-Signature – Could it be that multi-signature is only needed to create a transaction?

transaction include No signatures are collected. A transaction without a signature is different from a transaction with a signature. And without a signature, it is simply not valid.

For example, can I create a transaction using one of my UTXOs only when M signatures have been collected?

Yes, that’s literally what multi-signature output is. It can only be used in transactions that provide a sufficient number of signatures.

The transaction creator is responsible for collecting the required signatures from co-signers. Without them, there is nothing valid to send over the network.