Why the salt attached to magnesium changes absorption
Supplemental magnesium is always bound to another molecule, and that partner strongly affects how much is absorbed and how it behaves in the gut. Organic and chelated forms such as glycinate, citrate, and malate tend to be more soluble and better absorbed than magnesium oxide, which is inexpensive and contains a lot of elemental magnesium by weight but is poorly absorbed.
Poorly absorbed magnesium stays in the gut and draws in water, which is why oxide and citrate at higher doses can have a laxative effect.
Forms chosen for specific purposes
Some forms are selected for traits beyond raw absorption. Magnesium glycinate is often chosen for being gentle on the stomach. Magnesium L-threonate has been studied for its ability to raise magnesium in the brain in animal models and is marketed for cognitive support, though human evidence is limited and preliminary. Citrate is commonly used both as a supplement and, at larger doses, to relieve constipation.
This is general educational information, not medical advice. People with kidney disease in particular should not take magnesium supplements without clinician oversight, since impaired clearance raises the risk of harmful buildup.