Betaine HCl stomach pH
Betaine HCl (trimethylglycine) was present in over the counter "stomach acidifiers" but the US FDA says there is no evidence for its efficacy and has banned its use for this indication (source: US FDA) - it may be freely available elsewhere, I am unaware of the regulatory status in other jurisdictions.

There is no biological reason to expect the "HCl" portion for most drugs to have any substantial effect on stomach acid. Lots of drugs are packaged in an acidic form as "drug HCl" - this is usually because the acidic form of the drug is more soluble. Drug doses are commonly in the microgram to milligram range (though obviously this varies for different drugs) - at those concentrations, the stomach acid is much too concentrated to be affected by the small amount of acid added from the drug.

The catch with Betaine HCl is it looks like websites are advocating for taking several grams to provide enough HCl to affect stomach acid. I suspect that supplement makers suggesting these high doses is what led the FDA to ban the drug OTC in the first place. Although betaine is produced as part of normal metabolism, there are some side effects reported, mostly mild.

Note that anhydrous betaine is prescribed (and approved by US FDA) for homocystinuria, and used as a supplement to animal feed, but neither application is related to stomach acidification (and the anhydrous form is not acidified).

One recent study supports use of Betaine HCl with patients who are being treated with acid-suppressing drugs such as proton-pump inhibitors, to be given along with other drugs that require an acidic environment to function/be absorbed. However, it is important to note that this study was in fasted patients: they consumed nothing but a little water with the betaine HCl. In this circumstance, the little bit of HCl in a moderate to large dose of betaine was sufficient to lower stomach pH. However, the results likely would have been very different if the patients had eaten a recent meal.