Several creatures have the healing touch action, which gives some amount of dice plus a bonus of HP. Some examples below:
What is the bonus based on? It is not the proficiency or INT or anything like that.
There is no guidance on Healing Touch as a standard monster ability. All we can do is try to reverse-engineer the ability from the stat blocks that use it. Of the monsters you mention, the one formula I can find is half-proficiency bonus, rounding fractions up.
I also notice that all the creatures other than the Unicorn have a bonus equal to half the dice used.
There is a pattern established with the 5 creatures you link of half proficiency rounded up, but this pattern is broken by a creature from Eberron, the Undying Counselor, having a proficiency bonus of +4 and a Healing Touch of 3d8 + 5 (the Undying Counselor has a wisdom modifier of +5).
To be clear, this isn't explained anywhere, and the existence of a divergent data point makes it less clear. We cannot say "this is how it is supposed to be, and the example from Eberron is incorrect". We can only state factual observations about the existing data, as I have. Use this information however you like.
Notably, there is no guidance in the Dungeon Master's Guide for incorporating healing abilities into custom monster stat blocks.
Unfortunately, Wizards of the Coast have not graced us with an explanation on how they calculate that modifier value.
Without a statement from them on that, or in using an ability like that for CR generation, we simply can not determine what it is.
Humans are very good at finding patterns, but finding a pattern doesn't mean the pattern is there. Until WoTC provides an explanation, there is none.
If you were hoping for guidance in NPC development, unfortunately there doesn't appear to be any. As other answers, and you yourself, have noted, you can see how they've treated this ability at different CR levels to figure out how to create something with this. It's homebrew development, so don't be afraid to make changes after you've played it if it doesn't feel right.