Depends on the asset, most code or UI tools would probably be a no without a good amount of effort to port, 2D and particle assets you can most likely rip the image files and recreate in another engine with some work. 3D assets usually come with an FBX file (If they were made using Probuilder or something inside Unity you can use Unity's FBX exporter to get an FBX file) that you can easily transfer to another engine, there may be some edge cases where you'd have to re-rig the assets depending on the engine you're moving to. For animation assets they'll either be an FBX file that you can transfer over or an Unity Animation Clip that you can also convert to an FBX using the FBX exporter. Shaders are a little bit vendor locked if they were made with Shader Graph, you can get the generated source for the shaders, but it is generated which can make it hard to read (single letter variables/function names). There's a ton of edge cases you could run into depending on the engine you're moving too, like Z being up vs Y being up or engines using a different normal map tangent basis, but there's tools that can fix those issues when you come up against them.

There are a couple of projects working on this. These are two of the best ones that I know of:

* Unidot: https://github.com/V-Sekai/unidot_importer

* UnityPackage For Godot: https://github.com/barcoderdev/unitypackage_godot/

If I recall correctly there were 2 tools on github that made this possible i.e. unity assets to godot, but I don't remember their names. you can search on youtube and find them tho

This is such a vague question.

Yes, you can obviously export raw texture & mesh files into another engine.

Will the code work? No, probably not.