rotate object on slope surfaces / ramps - maintain snap on slopes

can someone please help me with this question (or point me to some detailed artilcles/examples of these rotator functions please )

I’m trying to rotate meshes on ramps/slope surfaces, however , I can’t seem to make the meshes snap to these slopes while rotating
As you can see here, the chair mesh aligns just fine to the slope , but when I rotate it left or right, it won’t take the slope into consideration

I have gone through quite a few topics on this forum & played around with many functions (Get Hit to get Normal point on surface…) but looks like I"m missing the core concepts of those rotator related functions (make Rot from X,Y, Z etc…)
and I"m just stuck here

Thank for your help!