How to clear cache in photos

Terence Devlin already explained about the working copies that Photos is storing for lossless editing and to speed up the browsing. They are essential. Some applications are offering to "clean" your Photos Library and to remove them. Apple is warning about them, because they will damage the library: Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

Older versions of Photos had a cache for syncing photos to an iPhone or iPad. That has been the only cache that could be removed safely, but doing so, would slow down the next syncing considerably. This cache does no longer exist in Photos 5.0 on Catalina, because iTunes Sync has been replaced by syncing with the Finder.

You need a new advisor. Or you need to listen more closely. It's really one or the other :)

Photos is a digital asset manager that is designed as a kind of digital darkroom. It is a parametric editor - that means that your original file is preserved and never touched. It's treated like a film shooter treats the negative. So, when you edit a snap the app notes the decisions you take in the database. If you export the shot then these decisions are applied to a copy of the original and you get the edited image as a result. But your original is preserved. This means you can always go back to the original and start over if you want.

For ease of use, and for the sharing mechanism, a preview is also created. That's a reasonable quality copy of the edited image and that's what you get when you send an image from Photos to an email say, via the sharing mechanism.

This is what Photos is designed to do. This is how it works. This is a feature not a mistake. There is no way to purge the preview or the original without significant data loss. Attempting to do so will corrupt your library, which will also lead to data loss.

None of this is remotely related to caching and no, you do not need to clear the cache, even if you do it will not recover an iota of disk space as it's all recreated the next time you launch the app anyway.

Make a choice: you want a photo management system that preserves your original like a film shooter preserves the negative, or you don't. Once you make that decision, all other things flow from it.

If you don't want this feature, that is if you do not want to preserve the original but rather what to edit it directly, the solution is simple: use some other app, one that works the way you want.

As for disk space: if at some point in the future library grows large, you can always just move it to an external disk.

Which photo cache are you asking about? Could you please provide some more information on what you want to do?