It’s a bit more complicated, but simply - haemochromatosis (HH) is an iron overload disorder (there are others)
In someone without HH, once the body has enough iron, it simply stops absorbing any more until it needs it.
People with HH don’t have that ‘stop’ switch, so the body just keeps on taking on iron.
The body doesn’t have a mechanism for excreting excess iron.
Eventually the blood is saturated & the body needs to stash the excess iron - in liver, joints etc
You use iron to make red blood cells, so when a unit of blood is removed from the body, you use up iron in replacing it.
The haemochromatosis society produces a very helpful information booklet, and there’s also information on the website of the British liver trust, and haemochromatosis society websites worldwide (notably Australia and Canada).
Worth bearing in mind that protocols for venesection, monitoring and levels etc vary somewhat not just country to country but within the uk, so your area might be a bit different from someone else’s.