Hi

They are "mason bees" making new babies.

They line the nest with leaves collect pollen and nectar, and then seal , they will hatch next spring the
"yellow substance "
will be the baby bee inside, they do no harm and lots of people buy the "Mason bee hives" to attach to a fence or post to encourage them into the garden.

Best just to leave them bee !

Gg

Well, I never knew that, Gnarly gnome, how interesting...

Yes, mason bees are solitary bees, and will nest in any suitable hole they can find. We currently have nests in holes in one of the house walls (screw holes made when putting up planters which have since been removed).

I may be wrong but I think mason bees don't have stingers.

you are correct they dont sting