there are so many surprising objects missing from SH3D Libaries

how does one create and add new ones? because i would like to add a lot (if i can)

i have seen talk of 'Blender' - which means absolutely nothing to me

pity we can't import all the huge number of such objects in Visio (which i previously used years ago, but MS seems to have just let die) - although i doubt it has the types of objects just for houses eg solar panels and tanks that i was just discussing.