In S/4HC, I've found no way to know exactly what was transported between Q and P in each change project. The View Change History - Details app in Manage Your Solution for the projects only give you the details of who created the project and when it was moved, not the details of what went into it. Yes you can add text notes, but that isn't systemic and wouldn't pass an audit if someone asked to see what config was made in x project (see attached for the level of details currently available). I view this as a pretty large gap in change management for the system, particularly if you have more than one person making SSCUI changes at a time, and we need better visibility.
I've created an influence request to address this lack of visibility and functionality and I'd encourage you vote on it to get it addressed soon: