Here's a list of some of the online resources where you can find .blend files to use as templates or to start from to create yours.
From blender.org
Demo Files This page contains several Blender projects from Blender Studio and from the community (usually licensed under a Creative Commons license). While not all of these are suitable as templates, many others are or can be used as a base to customize your template.
Some of the categories includes: Simulation, Geometry Nodes, Physics, Sculpting...
These are only a few, and of course you can search for other sources online...
Note
Remember to use templates compatible with your current Blender version. Some of the features used in templates are not avaliable in previous versions of Blender.