What statistical analysis should I use to compare demographic variables between different questionnaires?

I personally prefer Measurement Invariance. Measurement invariance, also known as measurement equivalence or consistency, is a concept in psychometrics that assesses whether a psychological instrument (e.g., questionnaire or test) measures the same underlying construct consistently across different groups, conditions, or time points. It is crucial in various research contexts to ensure that observed differences or changes are not due to measurement bias. Measurement invariance is evaluated through statistical analyses and involves different levels: configural, metric, scalar, and strict invariance. Achieving at least metric or scalar invariance is often necessary for meaningful group comparisons. Researchers typically use structural equation modeling techniques to conduct measurement invariance assessments.