When proposing changes to the Bitcoin protocol, there is a formal standard process called a Bitcoin Improvement Proposal (BIP). Bitcoin has no central leader or board of directors, so the community uses this open-source, consensus-based system.
Anyone can propose a BIP as an informal proposal. Historically BIPs are submitted on the Bitcoin email list, IRC or Slack. Some proposals can take years, others go through iterations and changes, some are ignored. Once a proposal has been adjusted and passed consensus it is then assigned a BIP number and published to the Bitcoin Core GitHub repository of BIPs. While it is not yet approved or implemented, developers can test it before implementing it to Bitcoin Core.
If approved by the developers and the Bitcoin community through consensus, the community will decide on an activation path and begin activating the BIP. This process has the goal of creating ultimate transparency driven by community decision-making. This is why running a node is important. The activation of these proposals is decided by the nodes that are on the network.