Noticed that some bees climb in at the bottom of my planters and dig themselves in. They go in and out multiple times a day, sometimes seem to carry some cut leaves. It seems like its the same bee, never seen more than one at the same spot, but multiple planters have these bees and holes.

What are they doing? Could they mean harm to my plants? Should I worry about my cat trying to catch them, and they stinging?

I also change the soil and the plant every year. Will I ruin their homes this way?