How does our brain get its Mg when we don't take Mg threonate which is the only form known to increase its levels?

Question

Obviously, if magnesium from foods or other forms of magnesium supplements can't enter our brains, our brains must be low in it before the invention of magnesium threonate. With low brain magnesium, plasticity is reduced and spacial memory is impaired.

But according to at least one study, magnesium threonate is the only form known to increase cerebrospinal fluid levels of magnesium. Other forms don't cross our blood brain barriers. https://www.sciencedirect.com/science/article/pii/S0896627309010447

Is it possible that:

1. During one of studies, the forms of magnesium weren't taken with meals which may affect the absorption
2. some forms of magnesium needs higher doses to reach our brains
3. the samplings were taken too early
4. magnesium threonate is naturally found in foods or it's formed during digestion
5. Losses from CSF samplings weren't taken into account
6. Results from rats may not always predict its effects on humans
7. Some forms of magnesium have laxative effects, depending on how they were taken, which may reduce absorption?

If we can get magnesium from foods into our brains, how does it work and what forms allow it to enter our brains?

If other forms of magnesium can't reach our brains, how did they improve sleep and reduce stress, both of which comes from our brains, in customers taking them?

If those issues were improved, can we be confident that our brain is taking in the magnesium from our supplements?

Here are some stories of Mg helping with those issues. https://www.psychologytoday.com/us/blog/evolutionary-psychiatry/201106/magnesium-and-the-brain-the-original-chill-pill

Answer 1

The article Magnesium transport across the blood-brain barriers (2011) from NIH.gov books says:

Magnesium is able to cross the BBB [blood-brain barrier]...

It is not the ScienceDirect article you linked in the question, but ScienceDaily article that mentions that

magnesium-L-threonate...effectively crosses the blood-brain barrier

The article mentions magnesium-L-threonate as one of the magnesium formulations that can effectively cross the BBB. This does not mean that no other magnesium formulations can cross it, but that some of them may not (efficiently) cross it.