The rules of Bitcoin (the protocol) and their enforcement by the Bitcoin network is what determines bitcoin’s (the asset) monetary properties.
The rules generate the characteristics of bitcoin’s properties, such as its scarcity resulting from the given supply policy, or its divisibility resulting from how many decimals the protocol outlines. These rules are then actively enforced by the network of users (full nodes, or peers) running the Bitcoin software.
Enforcement takes place via full nodes refusing to accept and relay transactions or blocks conflicting with the protocol rules, meaning that any users running software with incompatible rules, or attempting transactions that don’t conform to the protocol, will find themselves isolated from the Bitcoin network.
The protocol rules are intentionally hard to change. While anyone can propose changes to the Bitcoin software, other users do not have to accept these changes and may keep running the rules they originally accepted for as long as they wish. This ensures any rule changes must either not conflict with the previous rules or otherwise be acceptable to the vast majority of users. The Bitcoin protocol and network therefore work together to provide the characteristics of bitcoin, the asset.