Frame size?

Now you can have 2 bicycles with the same seat tube but bike A has 58cm top tube + 120 Stem and BIke B has 60cm Top tube + 100 Stem which both have 70cm reach. But is that the same regarding bike fit?

Roughly speaking yes.

But....

The position of your saddle relative to a vertical line going through the centre of the BB is constant for a given rider. A plum line hanging from your knee, should meet the pedal axle, when the crank is in the forward horizontal position.

So on a bike with a steep seat tube, you will need to push your saddle back. This will increase reach, for a given tt length. Whereas on a bike with a slacker seat tube angle, you will need to push the saddle forward, to achieve the same fore/aft position relative to the BB.

It is not advisable to adjust your saddle fore/aft to fit your reach. Rather to adjust your reach to fit your saddle fore/aft.

It also worth bareing in mind that the manufacturer will be assuming a stem length of 100 to 120 depending on size. If you deviate from this too much, your weight will not be positioned in the manner the manfacturer designed the bike for.

Bike fit is a complex business, if done correctly. Which is why people dont mind paying for an expert to fdo it for them.