What a BIP is
Changes to Bitcoin Core and the wider ecosystem go through an open process; [[en/books/inventing-bitcoin|Inventing Bitcoin]] names Bitcoin Improvement Proposals (BIPs) as the pathway for ideas — reviewed in public, with code and discussion visible to anyone.
A BIP is a design document, not law: nodes choose what software to run; economic actors decide what they call “bitcoin.”
Source: Inventing Bitcoin — Ch. 8
Activation in practice (examples from the blocksize war material)
The sources describe miner signaling thresholds (e.g. 95% of blocks in a difficulty period) used for several soft forks before SegWit — including BIP 66, BIP 65, and the BIP 68 / 112 / 113 bundle — and note that imperfect activations (miners signaling without actually enforcing) caused short chain splits until the network converged.
SegWit activation politics led to BIP 148 (UASF) — controversial because it risked consensus divergence if miners did not cooperate.
Source: The Blocksize War
Relation to forks
Soft forks are often specified and deployed with BIPs; hard forks may also be proposed as implementations (e.g. BIP 101 / Bitcoin XT era), but require every participant to upgrade for a single chain — otherwise two assets emerge.
See forks and governance.