Some of these insects appear to be fungus gnats, which are very common indoor nuisance pests. The others that look smaller we can't see clearly enough to identify. Possibilities include springtails, drain flies, and phorid flies. If you have any aphids, scale insects, or other typical houseplant insect pests, some of these might actually be beneficial parasitoid wasps, which can colonize a plant while outdoors and continue to breed for a time indoors if conditions suit them. If so, these parasitoids are welcome because they kill plant pests and don't bother people, pets, or the plants themselves. They tend to die out and disappear as their host insect declines.

It would help narrow-down the possibilities to know where they are being found. Most indoor flies will be breeding in sources of moisture, which would mean either damp potting soil (if a houseplant is being overwatered), inside drains, or in any other location with damp organic matter (decaying leaves in a gutter, stored bags of soil or compost, etc.).

We're happy to take another look if you can take clearer close-up photos, but we realize this is challenging without specialized camera equipment. If you have a magnifying glass, you can try using that. Plus, using a ruler next to the insects will not only help us judge their size, but its inch/millimeter markings will give a camera something finely-detailed to focus on when it has a hard time focusing on the insects themselves.

Miri