For people who can't or don't want to watch a yt video covering this:

https://www.bbc.com/sport/football/67695345

Premier League clubs have voted to limit the period over which a player's transfer fee can be spread in their accounts to five years.The move brings English top-flight clubs in line with Uefa regulations which were altered in the summer. It also prevents a practice noticeably used by Chelsea to strengthen their squad during recent transfer windows. The Blues gave lengthy contracts to new signings in an attempt to comply with financial rules. The Premier League said: "Going forward, a five-year maximum will apply to all new or extended player contracts." Clubs also approved a rule change to allow the league's board to block clubs from registering new players where they owe a transfer debt to another Premier League or English Football League (EFL) club until the debt is paid.

So that's how Chelsea spent so much (and gave such long contracts), they just spread the transfer fee over 8 years