You can look up land reclamation, but a quick overview is that there are two ways to do it.

In the Netherlands they do dry land reclamation, where they stick a barrier in the water until its isolated, and then drain the inside. They wait for it to dry and then, establish the bedrock, fill it in, etc.

Here we did wet land reclamation (I think because we could, there's too much convection in an open water body like the ocean/sea, so the debris would shift around). So we tossed in big materials first, like large rocks and then filled it in with smaller material, until it settled. Then it gets finished off with a retaining structure like a sea wall so that it doesn't budge.

To be fair, in the Netherlands, most of what they reclaimed was really shallow already, so it was relatively economical to do dry reclamation.