Berkshire does have a huge number of holdings and could be used to invest in a lot of different stocks in a single investment. But it’s not the same as a trust.
First up, investment trusts are a UK-specific variant of closed end fund. The US regulator instead just allows vehicles called closed end funds, which are basically similar but regulated differently to trusts in a different environment.
So how is Berkshire Hathaway different from US closed end funds?
US closed end funds are regulated in a specific way by the 1940 Investment Companies Act that requires them to treat dividend income in a particular way, defines how they can market themselves and use capital.
In particular, funds just pay their dividends out, without retaining that cash in the company. If you wanted to reinvest you could, but you’d really just be buying more stock of the trust.
This has implications for the tax they pay and the impact of that on investors.
Berkshire Hathaway is more like a massive conglomerate. This means it has more autonomy than a closed end fund. It pays corporation tax as a company would. It can choose to retain all the cash it generates - in fact BH has never paid a dividend (I think).
It fully owns and operates several subsidiaries, including many insurance firms which run under the Berkshire brand. You don’t pay a management fee as you would with a fund.
I don’t know of any closed end funds that outright own companies as subsidiaries - although they technically could.
So Berkshire, although you might use it for similar strategies to trusts, isn’t a trust or a fund.