You will find a good introduction to the field of mixed methods research designs in the textbook by Creswell and Plano-Clark: Designing and Conducting Mixed Methods Research (3rd ed.).
As you can tell by the fact that this is a third edition, the field of mixed methods research has grown rapidly, so they devote a chapter apiece to each of the most commonly used research designs in that field.
When you use two or more different methods of data collection and analysis. For example quantitative and qualitative methods used in one study. This is usually done for added validity and credibility.
This is quite useful for advanced researchers who have a good experience and have spent many years doing qualitative and quantitative research. Comprehensive reading, analysis and synthesis involving a qualitative study of papers published in a research discipline provides a good theoretical foundation. This is followed by a quantitative study based on a response to a questionnaire and analysis to build a construct or model. You may also use various problem solving techniques in your area of specialization to complete your goal. It is also good to read some text books on research methods that describe in detail about mixed methods research design.
Mixed methods research designs are approaches that combine both qualitative and quantitative research methods in a single study, aiming to provide a more comprehensive understanding of the research problem. They leverage the strengths of each method and can be used to validate, enhance, or explain findings from different perspectives.