is from the earliest mining maybe the sold some to cover up the electricty an hold the rest unless they have free electric but still
maybe one or two people not sell any single bitcoin they have
May be apart from Satoshi who mined the early coins have not moved the coins and who knows how many coins he mined apart from the known wallets and even he might have sold the coins and everyone else would book their profit at some point. Anyone who invested millions of dollars for a mining farm will not be holding the coins without holding as they will be booking their profit once in a while, if not they cannot sustain.