Steam bent. I see half a dozen shapes so there must have been half a dozen jigs to hold the wood for cooling & drying. The far greater puzzle will be the elegant planning of the layout.
Water in wood is in two places: free water in the open volumes of the cells and 'bound' water which is stuck to the cellulose and other wood molecules. Air-dried, under cover, outdoors, wood dried down to an equilibrium moisture content of 12-14 % is OK.
You need a steam generator connected to a steam chest where the wood is. This coming summer, I'll build one= 20lb propane tank with no valve, 1/3 full of water and heated with the burner from my big lobster pot.
That goes directly into a hole in a plywood "coffin" for a steam chest. . . . I won't put a latch on the box lid, that will do for ventilation.I want to try to make kerf-bent boxes from 1" WRC. I can cut a joint in less than 20 minutes (10+ different geometries). but so far, I'm too impatient with the steam heating.
Steam Bending Theory: steam is hot (duh!) and it is that heat which heats the bound water in the wood.
That heat also 'plasticizes', softens, the non-fibrous wood chemicals. The wood is like soft plastic. Test a piece from time to time, may take a couple of hours. Out of the box and into the jig to cool down and set (overnight?) as it dries.
Your picture: Possibly oak of some sort. Judging by the way that the grain pattern flows from one strip into the next in the bottom 12+(?), must have been some wide board!