Hi to both.
The DEM resolution is inherently and directly tied to the Dense Cloud point density. For example - if your project was flown on 1 cm GSD and you process your dense cloud as Ultra High quality, this means that the resulting dense cloud will have an average point density (sampling resolution) of 1 cm (i.e. the points are on average 1 cm spaced from each other).
If you process your project on High quality (instead of Ultra High) then this point density goes down by a factor of 2. If you process it on Medium quality, the density goes down by a factor 4. Low is a factor 8 and Lowest is a factor 16.
The Build DEM step generates the highest possible sampled DEM based on the point density of the Dense point cloud. Meaning that if your project has a 1 cm GSD - if you process your project on Ultra High, the resulting DEM will have a default sampling resolution of 1 cm. High produces a DEM resolution of 2 cm, medium produces it on 4 cm, etc...
The next step is exporting the DEM from Metashape in any arbitrary resolution that you want. Such as 1cm, 2cm, 3cm, 4cm .. as long as it's a lower resolution than the DEM produced by Metshape. So if you have a 1 cm GSD project and wish to generate a 3 cm DEM - simply process a High quality dense cloud, generate a 2 cm DEM (default values) and export it as a 3 cm DEM geoTIFF.