When open BLENDER and select 'general' to start. It start with many windows '3D viewport', 'Outliner', 'Properties', 'Timeline', Which they all 'area'.
We can add or close area by 'right click', and onMouseOver/click on 'area' to select 'Duplicate Area into New Widow' to make new area or 'Close Area' to close it.
We also can use mouse by place it near the corner till you see '+' sign then 'left click' and drag it, the new area will show up.
Go to 'Properties Area', select 'Scene' and goto '> Units'.
The key is to set 'Unit Scale' to match 'Length'. (as if set 'Length' to 'Millimeters', 'Unit Scale' need to set to '0.001')
Also with 'Grid', need to set 'Viewport Overlays' at 'Guides', 'Scale' to match, which in case 'milimeter' to '0.001'.
But anyway, if you want to do work in 'Length' as 'Meters', that mean, nothing to do with all these.
In object that need to use 'Subdivision Surface' Modifiers, add 'Bevel' Modifiers before hand with 'Segments' = '3', might help to reduce beveling work.
generate > boolen
generate > remesh (smooth : octree depth 1-9)
deform > smooth connection (only smooth , factor = 1, repeat 100-300)
generate > decimate (collapse : retio =?)