Most species of bees are solitary ie each female makes her own nest. Solitary bees are shorted lived as adults ie having grown through the stages of egg, larva and pupa before becoming adults. In the UK there are about 270 different species of bees of which about 240 are solitary bees.
Majority of solitary bees nest in the ground. The female builds the nest herself by using her body to dig out a nesting chamber. She then adds pollen to the chamber and lays an egg. She then seals off this section of the nest before moving onto creating another chamber. Some species of bees in the UK that nest inside the ground create turrets over their nests, which are often very distinctive.
A number of species also nest aerially, mostly in old beetle holes often sealing the nests with a saliva like substance, mud, chewed leaves, resin or sections of leaves which they cut with their jaws. These species of solitary bees are the ones most likely to adopt to artificial nests in gardens. The solitary bee Ceratina cyanea, excavates its own aerial nest, mostly in bramble stems. This small metallic blue bee excavates out the pith of the bramble stem and nests there. What is even more unusual is that both the males and females also overwinter, hibernating in the stems.
There is also a species of solitary bee that nests in snail shells and three of these species live in the UK. They use chewed up leaves to seal off each section inside the empty nest shells and camouflage the shell in some way or other.