There's a Wikipedia page devoted to it. Suffice to say, the transactions are extremely murky and would lead you to believe that concealment was the purpose of such murky dealings. And there have been other investigations of it including by Meduza and BBC Russian Service. Many of Navalny's conclusions were borne out by these subsequent investigations. Practically speaking there's no concrete way of getting evidence from a closed system like Russia has. The only way would be through buying hacked data on the dark web and even then would not be conclusive given its origins.
But that's just about the palace. Putin's corruption goes a long way back. Anna Politkovskaya's book "Putin's Russia" is a very good source of how things worked in his early days. Anna was assassinated in a lift (coincidentally? on Putin's 54th birthday) two years after the book was published. Well worth a read, although grim as ****.
Playing the man and not the ball? Not your best work.
I suppose if you squint a lot and look through your fingers, you might be forgiven for thinking that a constitutional limit of two consecutive terms, being increased to four and reportedly to soon become six is only coincidence and has not got the faintest whiff of dictatorship in a 'political system' that has no real opposition and shamelessly high votes for the incumbent, then fair enough. I just can't stretch my credulity that far I'm afraid.