The joinpsbts command merges multiple distinct PSBTs, each containing different inputs and outputs, into a single PSBT. This consolidated PSBT includes inputs and outputs from all the PSBTs provided. It's important to note that no input in any of the PSBTs can be present in more than one of them.

How is this used?

This command is particularly useful when multiple parties are collaborating on a transaction, each providing their own PSBT. By joining these PSBTs, the participants can create a single, unified transaction that includes inputs and outputs from all contributors. This simplifies the coordination and finalization process of multi-party transactions, ensuring that no input is duplicated across the PSBTs.