I wasn't involved with the insurance side of things. I'll tell you what I know / think, but can't guarantee it's right. Typically with insurance it will also depend on what cover you take up - clubs could theoretically insure for any eventuality but in practice they don't because of the associated costs. This is based on what I think I know about our arrangements, which I'd expect would be typical.
Injuries on international duty
It's the responsibility of the national association to fund the player's wages while injured. I never saw a case of a player with a career-ending injury so unsure if there would also be some level of "transfer fee" compensation too. Possible, but not sure.
Club injuries
Generally speaking it's just tough. If Wirtz (or anyone else) gets a career ending injury then I THINK the club can claim wage costs for the remainder of their contract once it becomes clear his career is over, but that's it. If he's out for 12 months and then comes back, albeit not the same player, the club has to foot the bill for everything.
I believe there wouldn't be any compensation for lost transfer value (as the insurer could argue the player would have wound his contract down and left on a free, so his value would be £nil).
Death of player (or permanent incapacity outside of football)
However, the club will periodically submit a value schedule to the insurers. Basically the policy usually has a "squad value" written in (essentially an aggregate claims limit) and that needs to be allocated between players in the squad. The staff at Kirkby (used to be Ian Graham's team) would periodically re-value and split the limit, and it would also be done after transfer windows. Then in the event of the accidental death of one or more squad members the club can claim up to that value limit under the policy. So chances are West Ham would get a pay-out for Antonio if it's ever determined that he can't return to playing, although it will probably be a small amount given his age / contract status. They may also be able to claim something for his wages if his return is ruled out.
So if you think back to Cardiff and Emiliano Sala, if they'd properly registered him they might have been able to claim under the insurance for the transfer fees due to Nantes. But he wasn't properly registered (paperwork not submitted, by all accounts) and even if he had been they probably wouldn't have notified the insurers straight away in order for him to be covered under their policy - hence all of the court actions that have followed. It's also likely the insurers would have tried to wriggle out of paying given the circumstances of his death (i.e. pilot and aircraft weren't fit for purpose).
Player's personal insurance
There was also one fringe first-teamer I remember who had a personal policy in place - essentially a loss of earnings policy. I only know he was paying a premium because it was in his contract and was paid by the club as a benefit in kind, I have no idea what the coverage was.
