When these companies buy bitcoin, they don’t do it on spot exchanges(e.g. Coinbase, Binance, Kraken, etc). They do it on Over The Counter(OTC) platforms. For the simple reason that buying huge amounts of bitcoin on spot exchanges would easily shoot up the price, ending up with them buying lower amounts of bitcoin units; and they obviously wouldn’t want that. They would want to get as much bitcoin as they can with their money so they buy them directly, as so they would only have indirect effect on bitcoin’s price.