On the Frontiers page, on the top part of the screen you have a "search bar" where you can look for articles, research topics, people, events and more:
The general search will then allow you to filter by category. Clicking on a category (for example "Articles") will also give access to dedicated filters
As an alternative, visit the Journal Page of the desired journal and then go to the 'Articles' tab. This will give you access to a search engine for the journal's articles. You can search by title or keywords and/or filter by Article Type, Section (internal subdivisions inside a journal) or for article that are part of a research topic or not.