Violet, no idea which species.

Can it be Violet weed even though it has yellow flowers? What is fastest way to get rid of them?

Yes, violets come in pink, purple, white, yellow, some see some of them as blue. They are perennials, and heavy seeders.

The RU may not have been effective if it wasn't yet a very warm day, that's when it works best, by being processed by the leaves and taken into the core of the plants. Some plants aren't affected by glyphosate, violet may be one of them. Until then, you could weed-wack them, but violets can make seed pods that are so low or even under ground that this may be a waste of time. For that reason, that I think you might be able to kill the existing plants with RU but obviously the seeds were either there or came in with the soil or mulch or even the root balls of the landscape plants and more are now being created, it's going to be ongoing chemical warfare for years. If that's how you want to handle it, just try to get the most bang for your buck by making sure it does kill violets first, then explicitly following the instructions on the label about the best results, making sure the mist doesn't get on desirable plants, be thorough.

A little more work up front might yield better long-term results (if you don't plan to do much additional digging in the area in the next few years - which would allow seeds to sprout, defeating the efforts if one isn't vigilant about inspecting newly disturbed areas often for sprouts. That wouldn't be hard, but some people just aren't the type.) That would entail either raking the existing mulch aside to smother the plants, or smothering as it is, then adding more mulch.

If interested, please see this discussion and/or this discussion for more about smothering.

I was thinking that it might be Ficaria verna. Can you get a picture of the opened flowers?

The pictured plant is Ficaria verna, previously known as Rununculus ficaria, common name Lesser Celandine. It is not any kind of violet. In the photo the flowers are closed but they will open in sunshine and the similarity to its cousins the buttercups will be obvious.

Here is a link that might be useful: Lesser Celandine

Boiling water.... Cheap / easy.... Repeat as needed

Thanks, Flora, are they perennials as well?

Crscra, if there are roots of desirable plants in this area, that would be a bad idea. Otherwise, I agree, but got the impression it's a fairly large area...

They most certainly are perennials and massive invaders. They will disappear in the summer - but don't be fooled, they'll be back. Spread by little bulbils, as well as seed. A British native but invasive alien in the US.

Thanks again Flora! Sounds like it would be the same fight (and spreading process) as violets. I like native violets, but would go after this one as I described.