[TheCamper]

If this is a slab on grade house then the slab was a separate pour from the foundation and the crack is just where the slab meets the foundation wall. There is no way to avoid this "crack". If this is a basement slab where the slab was poured over the footing then it is a crack from the slab settling because of inadequate compaction and/or not installing wire mesh properly.

 

[TheCamper]

I agree that mesh doesn't add any substantial strength to the concrete but it does tend to hold it together and reduce cracking as a slab settles. This becomes especially important in a basement slab as the masons tend to pour a loose (weak) mix so that they can push it around easier. If this is a basement and the slab was poured extending over the spread footing, then it is likely that settling of the soil beneath the slab has caused the slab to drop resulting in a crack above the edge of the footing. The remedy is to remove your wood framed walls, break up the slab, compact the soil and pour again. You are not likely to do that for a crack or small difference in surface heights. You are going to fill the crack or level the floor with a concrete leveling product. Neither really solves the problem.

If this is a slab foundation then depending on what part of the country you are in will somewhat dictate if you can do a monolithic pour. Depending on your soil bearing capacity and the load imposed a spread footing is often required with the foundation being formed above. Any underground mechanicals are then roughed in and the slab poured. Additionally, if KNRE is up north (I may have missed the geographic location but I didn't see it), the slab is poured separately for a thermal break, typically with foam in between the foundation wall and slab.

KNRE: Are the photos of a basement floor or a slab construction at grade level? What part of the country are you in?