It's a little from column A and a little from column B.

berkshire hathaway

Berkshire Hathaway are value investors, meaning they buy stocks based on longer-term value. So if they buy a stock, that doesn't indicate that they think the stock will go up in the next month or even year, but that they think it's selling for a good price relative to earnings. In many cases, they'll completely buy a company (e.g. GEICO) because they think it'll perform better under their direction than just owning some shares. Also, Berkshire is rarely looking for the best returns (e.g. moonshots that pay out), they're looking for consistent, competitive performance. For example, Berskshire Hathaway doesn't own any Tesla stock because it's too expensive relative to earnings and they don't know where it'll be in 5-10 years (whereas something like Coca-Cola is more predictable).

Since they're not frequent traders, they're not really that involved in setting prices. However, they've had very consistent performance, so a lot of investors make investment choices based on how they invest.

managed by small number of individuals

The thing is, those individuals have competing interests. Some are in it for short-term gains (many hedge funds), some are in it for income generation (dividend-heavy mutual funds), and some are in it for long-term, steady growth (Berkshire Hathaway). And there's a lot of competition within each group, especially hedge funds, where they're trying to snap up customers by comparing themselves to their competition.

Prices aren't set intentionally by some cabal, they're set by this competition where one group thinks they understand the companies and market sentiment better than another, so they'll bet against each other and the net result is the stock price.

day-traders making option plays

There are two types of day-traders, there's the individual investor controlling somewhere between $100k and a few million, and then there are hedge funds that control hundreds of millions if not billions of dollars. The first group is much larger (in terms of people), and the second group is generally more influential because capital is more concentrated.

Here's a breakdown of who is trading.

Liquidity (i.e. trades):

• 30% - "naturals" - this is direct stock trading, like from hedge funds, individuals, pensions, etc
  • 7.7% - hedge funds
  • 6.4% - individuals/households
  • 5.3% - mutual funds
  • 5.1% - pension cash flows
• 70% - "intermediaries" - banks, options, futures, etc
  • 18.6% - wholesalers - largely off-market trades, like 401k custodians moving money from the company to the broker
  • 16.2% - market makers - often brokers that post simultaneous buy and sell orders to both increase liquidity (availability of shares) and profit from the bid/ask spread

Ownership of the stock market (as of 2019):

• 34% - households
• 22% - mutual funds
• 16% - foreign investors
• 11% - pensions and gov't retirement funds
• 6% - ETFs
• 4% - business holdings
• 3% - hedge funds
• 3% - other

So, from these we can see that households (you and me) control the biggest share of the market, but we do a relatively small fraction of the trading. So if retail investors (you and me) band together, we can manipulate share prices, like what happened with GME. And you don't need everyone to band together, you just need enough people to push demand for a stock higher than what other investors expect, and you'll get a bandwagoning effect (e.g. day traders, bots, etc all thinking there's a reason for the stock to move). GME started because retail investors at /r/wallstreetbets wanted to stick it to short sellers (in many cases hedge funds who thought the stock would go down, so sold shares they didn't have), so they pumped the stock to screw those groups over (short sellers have to repurchase at some point...).

massive firms, insurance companies, tech companies and other large holdings managed by small number of individuals impact stock price a lot more and with that don’t have an insensitive to sell holdings they bet big on.

They'll sell if they think their money is better invested somewhere else. Berkshire Hathaway just sold a bunch of Apple stock to free up capital for other opportunities, even though Warren Buffett still likes Apple (makes up >40% of Berkshire's portfolio).

I think a lot of people worry too much about these "mega investors," but at the end of the day, there's not really any kind of collusion. The SEC shuts down whatever collusion there is, and it's usually pretty small (e.g. in the millions, not billions, and the global stock market is $100T+).

The groups with the most sway over short-term stock movements are those who trade the most, and that's mostly hedge funds and mutual funds. However, longer-term stock movements are more impacted by the quiet majority who buy and hold, because that produces more demand for underpriced stocks and less demand for overpriced stocks, just by virtue of market cap weighting.

At the end of the day, a stock price tends to reflect the future expected value of the company, not the current value of the company (i.e. book value). That's because buying stocks is a mixture of finance (calculating book value + expected growth) and psychology (market sentiment for the company's products). If a company goes bankrupt, it doesn't matter how much collusion there is to pump the stock price, those shares will go to zero.

My holdings are primarily in ETFs

And mine are almost entirely index mutual funds (VTSAX and VTIAX and equivalents).

I enjoy looking at market trading, but the statistics show that, long-term, index funds beat active trading, and it's not even close. All of these active trades by hedge funds and active mutual funds may have the biggest impact on prices, but they don't seem to have consistent, long-term impact on profits, so it's honestly better IMO to leave the active trading and stock price setting to these funds instead of getting involved as retail investors.