A new operating system version reaches some phones immediately and others months later or never. The delay is produced by a chain of parties, each with its own incentives.
The chip vendor has to go first
Low-level software that drives the processor, modem, camera pipeline and other hardware blocks must be updated for each new system version.
That work is done by the chip vendor, and a manufacturer cannot begin its own integration until the updated components arrive.
Vendors support a given chip for a defined period, and once that period ends a device using it cannot readily receive further major versions regardless of the manufacturer's intent.
Manufacturer software must be rebuilt
Most Android devices ship with a substantially modified interface and a set of additional applications layered over the base system.
Each new version changes underlying behaviour, so those modifications must be adapted, tested and in some cases redesigned.
The effort scales with how far the customisation departs from the base, which is why manufacturers with lighter modifications consistently ship updates sooner.
Testing multiplies across variants
A single model may exist in several regional versions with different radio configurations, and each combination has to be validated.
Radio software in particular requires certification against network operator requirements, which is a formal process with its own timetable.
A manufacturer with a large catalogue must repeat this for every device it supports, and prioritisation naturally favours recent flagship models.
Carriers add a further stage
Where phones are sold through network operators, the operator may test and approve builds before distribution, and may include its own applications.
This adds weeks to months and explains why an identical handset bought unlocked often updates well before a carrier-branded one.
The gap has narrowed as operators reduced their customisation, but the approval step remains in place in several markets.
Support commitments changed the calculation
Longer guaranteed support periods have become a competitive feature, and meeting them requires keeping older devices in the testing rotation for years.
That commitment has pushed manufacturers toward architectures that separate vendor components from the system, so the low-level layer need not be rebuilt for every release.
The structural change matters more than any single promise, because it reduces the cost of each update rather than only the willingness to fund it.
Security patches follow a separate and faster path, which is why a phone can receive monthly fixes for years while remaining several major versions behind.