File dialogs - such as Open, Save As, Insert text and the like - are available in two different ways:
As native file picker dialogs of the window manager of your operating system.
As built-in LibreOffice file picker dialogs.
Use LibreOffice - PreferencesTools - Options - LibreOffice - General to shift from one to the other.
Folder selection
Pick up your preferred folder from the pull-down list or type its path name. Autocomplete function can be used to ease typing.
Connect to a server using the File Services dialog. Select a parent folder from the folder path with .
Add a subfolder to the current folder with .
File name
Enter a file name or a path for the file. You can also enter a URL
File type
Select the file format for the document that you are saving. In the display area, only the documents with this file type are displayed. File types are described in Information on Import and Export Filters.