A visible problem does not always show where the fault begins.
A cart error could involve theme code, an app or product configuration. A layout problem may appear only with certain content or screen sizes.
I begin by reviewing the affected pages and trying to reproduce the issue. Recent changes, error details and the steps leading to the problem help narrow down the investigation.
The proposed fix follows what we find. If the issue needs more investigation before it can be estimated, that is explained as part of the scope.