This was my biggest worry too, and the answer turned out to be regular commitments rather than one-off socialising. A weekly class, a volunteering slot, a walking group — showing up to the SAME thing repeatedly is what turned acquaintances into friends at our age.
Younger expats meet people through jobs and classes; we retired here and the days can be quiet. We're not shy, but the ready-made social scaffolding just isn't there at our stage. For other retirees — how did you actually build a circle of friends from scratch in your new country? What worked beyond 'join a club'?