Boundary from Points

In QGIS, many of the really good tools are in the processing toolbox; you need 'concave hull':

Try it with different threshold values for different levels of detail:

Finally, add a 10% buffer around the outside to make it resemble the sketch you provided:

Try Concave hull, What are Definition, Algorithms and Practical Solutions for Concave Hull?

Concave hull has a smaller area, and most of implementations allows you to tune how small and precise resulting polygon should be.