On Oiler's salary cap page, the number for the estimated salary expenditure for 2021 is $85,603,497. How is this number estimated using the numbers shown for the Oilers roster? Anyone?

If I've understood this right, the total cap hit is totaled by the per-day (might be per game, but iirc, daily hits are calculated on a 183-day season) cap hit of each player not in the minors or on the taxi squad (up to $1.05M per contract; beyond that total the money counts). You get that $85+M figure by totalling the daily hits of every Oiler that's been on the roster throughout the season. When you open up a player's page, it'll show you their accumulated cap hit. Sum those and you get a team's total cap hit.

To understand how the Oilers - and any of the other teams currently well above the $82.5M cap ceiling - is to factor in their LTIR pool Edmonton's LTIR pool consumes Klefbom's entire hit, the percentage of Koekkoek's hit when he hasn't been active on the roster, Kassian's percentage from when he went onto LTIR, plus any other players that have been on LTIR or bounced between the taxi squad.