It is sometimes seen as a 'third' approach, which combines qualitative and quantitative methods systematically within the same project, while reflecting on the ontological and epistemological assumptions in those methods. There is a good discussion of the approach here: Timans, R., Wouters, P. & Heilbron, J. Mixed methods research: what it is and what it could be. Theor Soc 48, 193–216 (2019). https://doi.org/10.1007/s11186-019-09345-5

There are a number of high quality textbooks on mixed methods research, including that by Creamer and by Creswell & Plano-Clark.

I personally find the article by Timans et al. to be unreasonably critical because it holds mixed methods research up to standards that were never claimed by practitioners of mixed methods themselves.

So-called mixed methods is 'exactly what it says on the tin'---a combination of data collection methods within a research project; commonly understood as a combination of quantitative and qualitative methods, but this is not necessarily so; can also literally be a combination of EITHER qualitative methods or quantitative methods.

The essential rationale for mixing methods is that this might lead to greater understanding of a topic than if one method was used alone.

Mixed Methods Research uses a combination of qualitative and quantitative data collection and analysis approaches into a single study. It has three core designs: Convergent design, explanatory sequential design, and exploratory sequential design. For details on each, you may go through Creswell and Clark’s (2017) book.

Creswell, J. W., & Clark, V. L. (2017). Designing and conducting mixed methods research (3rd ed.). SAGE Publications. https://study.sagepub.com/creswell3e