Do not trust, verify!
Not your keys, not your coins.
So... exchanges, ETFs and Saylors have as much coins, as they publicly proved their addresses. Else it's just charlatanery.
It takes a bank 3 years and about 4000 man days to implement a Bitcoin (and shitcoin) trading and custody solution in the tradfi software.
Remember, this have to be paid.
16 years, devs paved the way so you can have your #Bitcoin solution with just 12 words... for free.