none except serpent is not used in everyday speach

Yes of course 🌷

Hello. I think snake means a snake in total, but serpent is a very large snake. Snake refers to a kind of animals

In that context, they mean the same. Serpent means the same thing, but is often not used. Snake is the most commonly used term, serpent you may here on occasion. For everyday speech, snake would be most appropriate.

Yes, the are basically the same. SNAKE can also be used as a verb. Example: The winding river snaked though the lowlands.