RegionPlot3D[x^2+y^2≤1&&0≤z≤Sqrt[3](1-x^2-Abs[y])/2,{x,-1,1},{y,-1,1},{z,0,1},AxesLabel->{x,y,z}]

f[x_]:=x^2
g[x_]:=0
RegionPlot[g[x]≤y≤f[x],{x,0,5},{y,0,25},AspectRatio->5]
RegionPlot3D[g[x]^2≤y^2+z^2≤f[x]^2,{x,0,5},{y,-25,25},{z,-25,25}]

f[y_]:=Sqrt[y]
g[y_]:=0
RegionPlot[g[y]≤x≤f[y],{x,0,5},{y,0,25},AspectRatio->5]
RegionPlot3D[g[y]^2≤x^2+z^2≤f[y]^2,{x,-5,5},{y,0,25},{z,-5,5}]

f[x_]:=x
g[x_]:=x^2
RegionPlot[g[x]≤y≤f[x],{x,0,1},{y,0,1}]
RegionPlot3D[g[x]^2≤y^2+z^2≤f[x]^2,{x,0,1},{y,-1,1},{z,-1,1},
PlotPoints->100,WorkingPrecision->60,
Method->{"TransparentPolygonMesh"->True}]

f[x_]:=x
g[x_]:=x^2
RegionPlot[g[x]≤y≤f[x],{x,0,1},{y,0,2},
PlotPoints->50,WorkingPrecision->32,
Method->{"TransparentPolygonMesh"->True}]
RegionPlot3D[(2-g[x])^2>=(y-2)^2+z^2>=(2-f[x])^2,{x,0,1},{y,0,4},{z,-2,2},
PlotPoints->50,WorkingPrecision->32,
Method->{"TransparentPolygonMesh"->True}]

f[x_]:=x
g[x_]:=Sqrt[x]
RegionPlot[g[x]≤y≤f[x],{x,0,1},{y,0,2},
PlotPoints->50,WorkingPrecision->32,
Method->{"TransparentPolygonMesh"->True}]
RegionPlot3D[(2-g[x])^2>=(x-2)^2+z^2>=(2-f[x])^2,{x,0,1},{y,0,4},{z,-2,2},
PlotPoints->50,WorkingPrecision->32,
Method->{"TransparentPolygonMesh"->True}]