Which hearings? If you mean the ones on the gantry side rods those are held in by the pulley grub screws. The lead screw ones are hend by gravity on the upper bearing, and a collar on the lower (but the lead screw could be machined with a thread too and go to a nut which would be even better. The bronze bushings on the rod end carriages and central carriage are held in by ABS softening with acetone and pressed in (assuming you printed in ABS). Otherwise if you use PLA or PETG for those parts use a glue of some sort, but try not to make the fit too tight or you can split the part due to how it is printed.
Yes there is pulley/gear reduction on the gantry and z stages. I go with 20tooth on the motor, ad 32tooth on the rods. But others have played with different ratios too. I just used what the original Eustathios by @Jason_Smith designed and it works great of me. I have found 16tooth pulleys to have less tooth engagement than I would like so stuck with the standard pulleys. But 32/16 reduction would give integer values on the reduction which might be nice on the Z-stage to get full step optimal layer heights to equal more standard layer sizes (.1mm, .2mm, etc.) Right now the full step optimal layer heights are easily calculated, but do not work out to be nice round numbers: http://prusaprinters.org/calculator/