QGIS has a lot of options to customize how it looks and feels. It can therefore sometimes be difficult to follow instructions/demos if it all looks different. There are three main areas you need to be aware of:

• Changing the User Interface Language.
  QGIS has an, in many ways, recommendable approach to language. By default, it uses the language of the system it is running on. However, many learning resources are in English, so it is probably easiest to learn QGIS by changing its language to English. Read how to here.
• In QGIS panels (internal window with in QGIS) and toolbars can be relocated to even closed. Read how to reopen and move these elements here.
• Extending/hiding functionality using plugins. The functionality of QGIS can be extended using so-called plugins that can be installed within QGIS. Even a brand new QGIS insterlation will containing som plugins by default but these can (typically by acedent) be hidden. The classical problem is here that the entire processing menu is removed). Read more about installing and hiding/unhiding plugins here.

You can also watch my QGIS UI issues