OpenCore, explained.
Apple stopped updating your Mac. A few thousand volunteers didn’t.
Every Mac has a last macOS — the final version Apple will officially install. After that, the security updates taper off, then stop. The machine still works. It just stops being invited.
OpenCore Legacy Patcher is a free, open-source tool that forges the invitation. It convinces a newer macOS that your older Mac is supported hardware, then patches in the drivers Apple left out. A 2015 MacBook Pro that officially ends at Big Sur can run macOS Tahoe. A 2018 MacBook Air stuck on Sonoma can keep going.
“The machine still works. It just stops being invited.”
What you’ll need
The short version
Time Machine or a full clone. OpenCore is reliable, but this is still an operating-system install.
Download OpenCore Legacy Patcher, point it at your USB drive, and let it fetch the macOS you want.
Boot from the drive, install macOS as usual, and let the patcher apply the drivers your Mac needs on first boot.
Updates arrive through Software Update like always. The patcher re-applies itself after each one.
This is unsupported territory — Apple won’t help if something breaks, and some features (DRM video, Handoff) can be temperamental on the oldest machines. Back up first. Then enjoy the extra years.