Besides the cellulose fiber in the wood cell walls, there are a lot of other wood chemicals which soften with heat.
When those things are heated enough, the wood can be bent, cooled in service and you will have the shape that you need.
Wood contains water in 2 places: the 'free' water in the cells and the 'bound' water which is another part of the wood cell wall construction.
The steam is a convenient way to add heat to the bound water which heats and thus softens the wood chemicals so that the wood can bend. Nothing magical about it. Boiling water would have the same effect.
The process takes time, you can't very well rush it because you have to heat the entire thickness of the wood. In direct answer to your question, yes, you can boil it or even pressure-cook it if you had to go that far.
The wood: green wood would bend most easily as it contains the most water but there will be a long, long cooling and drying process. Air-dried wood has a Moisture Content of 12-14% (approx) and most of that is the bound water, so that bends OK but takes time. Kiln-dried wood is the most difficult because a lot of the bound water has been heated and driven off so the heating/softening part may or may not work well at all.
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