Thanks to NASA’s Juno mission, we now have a few clues as to what Jupiter’s internal structure looks like. At its centre, extending out by up to 30 per cent of the planet’s radius, is a dense, liquid core made of ionised (‘metallic’) hydrogen and helium, mixed with dissolved heavier elements. The pressure and temperature inside Jupiter drop off as you get further from the centre. This means the liquid interior eventually gives way to a gaseous atmosphere (also mostly hydrogen and helium).
The depth of this liquid/gas boundary is not well defined, but Jupiter is probably fully liquid a few thousand kilometres below the planet’s cloud tops. So, even if we stripped Jupiter of its gases, it would still be bigger than Saturn.