Happy new Year!
A great way of learning about bees and insects is to have a close look at what other critters are using the nesting tunnels besides solitary bees.
This is a photo of routered tray that has been used by insects. You can see the green lining and the green cell divisions, made from chewed leaf materials. Inside each compartment is a cocoon containing a hibernating bee. The bee may be fully or partially developed. Some species overwinter as a pupa and develop into the adult bee the following spring/summer. The yellow/orange pellets are fecal droppings, and the yellow wash is pollen not eaten by the developing bee larva.
These "summer mason bees" come out and pollinate any time between May and September. Each species is around for about a month. They usually use a smaller diameter nesting tunnel than the spring mason bee Osmia lignaria. The nesting tunnel diameter used is anywhere between 3/16" to 1/4"or 4-7.5mm, depending on the size of the species.
The insect inside the lower tunnel, is a fly! A bee has two antennae. A fly does not. Flies do have a hair like structure, but is not visible in this photo.
Photo by Mike N.Vancouver BC