meaning of 'so much of'

I was reading this article: https://www.independent.co.uk/news/health/plants-prescribed-depression-anxiety-manchester-trial-a9078041.html.

It is about a medical practice prescribing planting instead of drugs. A GP commented: 'So much of what keeps people happy and well isn’t medical. That’s why ideas like this one are so wonderfully effective...'.

I am not sure what 'so much of' here means. Does it mean this method is very rare?

I cannot find the exact phrase in dictionaries. A similar phrase I found is 'so much for', but the meaning of it does not seem to fit here.

“So much of what keeps people happy and well isn’t medical."

I am not sure what "so much of" means ...

Split the sentence into its parts.

“So much of | what keeps people happy and well isn’t medical."

It's a quantifier, it specifies the amount. It's more than none and less than all; presumably the author is implying that the amount is significant, thus supporting their argument.

Alternatively:

• "Some of what keeps people happy and well isn’t medical."

• "A lot of what keeps people happy and well isn’t medical."

The exact quantity isn't made clear, that makes the statement more difficult to disprove.

Cambridge Dictionary - "Much, many, a lot of, lots of: quantifiers from English Grammar Today:

We use the quantifiers much, many, a lot of, lots of to talk about quantities, amounts and degree. We can use them with a noun (as a determiner) or without a noun (as a pronoun).".

So much, so many with a noun

We use so rather than very before much and many in affirmative clauses to emphasise a very large quantity of something:

He has so much money!

Not: He has very much money!

There were so many jobs to do.

Much of, many of

When we use much or many before articles (a/an, the), demonstratives (this, that), possessives (my, your) or pronouns (him, them), we need to use of:

How much of this book is fact and how much is fiction?

Claude, the seventeenth-century French painter,
spent much of his life in Italy.

Unfortunately, not many of the photographers were there.

How many of them can dance, sing and act?

Lexisrex.com - English Sentence Analyser - "So much of what keeps people happy and well isn't medical.".

If the context isn't very long it's best to include all of it:

Dr Ruth Bromley, GP and chair of Manchester Health and Care Commissioning, added: “So much of what keeps people happy and well isn’t medical. That’s why ideas like this one are so wonderfully effective, building on what is best about our communities and supporting patients close to where they live.”