Just continue to water. It's likely that you have a winter annual, probably chickweed, that is germinating. Winter annuals germinate in the late summer/early fall. Once you start mowing your new grass you will also be cutting a lot of the weeds which will keep them in check somewhat. When the grass is mature enough you can use herbicides to kill weeds. If you had used Tenacity at seeding time you probably wouldn't have much weed pressure, live and learn I guess. If the weeds are very dense then you might have some issues with it preventing your new grass from growing. Time will tell.

Thanks for responding. I tried pulling some up by hand but I think grass is coming up with it.

Can I gut the leafs of the stem with a pair of scissors and will that kill it so the grass will have a chance to grow through?

This pest is like at the surface level and
growing like wildfire.

This is another reason to not bring in topsoil. If you ran the round up twice with daily watering in between, you should not have any weed seed left in your soil. The fact that you do leaves the only possibility being the new topsoil.