PEAK uses P2P networking as far as I can tell. AmongUs has multiple mods that allows P2P gaming[0].
Most indie games don't rely on servers for the gameplay loop because those are expensive and an indie studios can't afford it. They instead rely on P2P networking for the gameplay loop, and at most have a matchmaking server to both give a nice experience to find other players to play with, and to allow NAT traversal.
You can easily design a game like PEAK/AmongUs in such a way that, even if the matchmaking server is disabled, the game still works because it has a direct connection or LAN option.
PEAK is listed as an example of using the Photon Fusion Shared Authority model[0]. AFAIK Photon Fusion doesn't allow for any proper P2P in any of their network topologies (apart from the fully offline single-player mode).
Most indie games don't rely on servers for the gameplay loop because those are expensive and an indie studios can't afford it. They instead rely on P2P networking for the gameplay loop, and at most have a matchmaking server to both give a nice experience to find other players to play with, and to allow NAT traversal.
You can easily design a game like PEAK/AmongUs in such a way that, even if the matchmaking server is disabled, the game still works because it has a direct connection or LAN option.
[0]: https://github.com/InvoxiPlayGames/AmongUsP2P