I don't understand the difference between users custom fields vs articles custom fields
can you explain in more detail what is missing ?
maybe with a screenshoot or whatever ?
thx

If you add custom fields to articles, the Joomla standard search function looks at the contents not only of the articles, but of the related custom fields as well.
This does not happen in custom fields that are related to users, the users search ignores the content of coupled custom fields.

There is no search plugin for com_users

I see, confused the search field above the users list with a real search.
But would it not be a great idea to have a real search on users+user custom fields?
It could be the start of a mini CRM, membership registration, just to name a few!

It sounds like you're mixing two separate things here? You can't filter in the admin list views for items matching a custom field value (the backend has never had "real" search, "real" being very subjective here), the frontend search is the only bit that matches this. And since com_users has no frontend views, obviously you can't search for anything there matching the component.

This sounds like another case of "I thought custom fields worked as a CCK", which has been a recurring opinion since the component was merged, and annoys people when they're told "no, it doesn't work this way".

Probably I did mix 2 different things.
Obviously the front-end search as we have now for articles does exist for users.
So to make my idea ever possible the search engine should be extended to users + their custom fields?
And frontend views should be made for users. I guess that's far off, right?

Frontend views for users can already be done, they need to be coupled to a Contact. Convoluted I know, but that's how it works. With the "Contact Creator" plugin, a new Contact is automatically created for each new user.

@EricdeWaal Thank you for your request. I understand, that this would be interesting for you, however we decided we wont be implementing such a feature into the Joomla core. We've added the possibility to search through custom fields in the frontend with smart search in Joomla 5 in general, however we wont be adding a feature to search through the user data in the frontend. This is a data privacy nightmare waiting to happen and to be honest, I'm not willing to take that chance. However, you can implement that for your own site in a custom plugin for smart search.