I would like to search content depending on select menu. Something like the search box here: http://www.doyleia.com/anazitisi-ergasias How can I do that with drupal 8?
You can achieve that with filters in Views.
Just create a view, add filters and make them "exposed", so that visitors can choose filters themselves. Filters can be taxonomy terms, list options or open-ended fields (e.g. text field) where users can write what they are searching for.
See the Chosen module for providing a more user-friendly interface for
<select> elements that accept multiple choices.