You can add a custom apt repository (something similar to Ubuntu’s PPA) to your operating systems. Check this link for details: https://apt.syncthing.net/
This repository is maintained by the syncthing team itself and always contains the latest builds (release/stable and candidate versions).
Syncthing started with version “0.2…” and then increased the second number on major updates. The leading “0” generally has a special meaning in semantic versioning, basically meaning that the build is “beta-like” (non-stable API). The versions increased up to the “0.14.x” line and when “0.14.55” was due the developers decided to simply call it “1.0.0” instead and finally drop the leading zero. See the announcement for details.
Versions with appendixes like 0.14.43+ds1-6 are usually made by third party distributors, for example by the official debian repository packagers.