Do search engines always use the User Agent?

Question

Search engines

Greetings

Is it always and without fail search engines (robots, spiders, their various services) are presented in the user agent or there are cases when, for example, google or another search engine or one of their services may not be presented in the user agent ?
In other words, can I unambiguously select a search engine through the user agent or are there exceptions?

Answer 1

There are many people who claim that they were rescued by dolphins, but I have not seen one who would say that he was drowned by a dolphin.

Answer 2

Any user can install any user-agent. So, it is impossible to determine unambiguously.
As for bots and search engines, there are many of them. It is better to find out directly from the search engines themselves.

Answer 3

The search engine puts the user agent at its discretion, you can’t guarantee it in any way, it’s like saying that a person only walks along this street.