Nazi Germanisation of European countries can come under two main categories: countries that were already Germanic with similar cultures/races to Germany, and countries that were viewed as suitable for the German race but were populated by "inferior peoples" (eg. Lebensraum).

Starting with the first category; the Nazis never really came up with a solid definition of Aryan per say, as under a wider definition countries like Iran, India, or Pakistan could have been considered Aryan yet I doubt Hitler would have enjoyed these people wandering the streets of Berlin. Therefore, the term Germanic tends to be more useful when differentiating what Hitler would have actually meant. Generally, the Germanic nations of Europe during WWII were: Germany (and Austria), the Netherlands, Luxembourg, Liechtenstein, Denmark, Sweden, Norway, and Iceland. Countries such as England, Finland, Belgium, Switzerland, and France were considered to be either semi-Germanic with a large part of the population of Germanic origin, or considered honorary Germanic for various reasons. These countries would have been considered fit for Germanization by the Nazis due to the fact that their cultures were already similar to that of an ideal Nordic German, and also these countries were populated by "Germanic Aryans" who were desirable by the Nazi state for there genes. However, out of these countries that fit the criteria for Germanisation, the Nazis did not plan to immediately annex most of France, England, Sweden, Switzerland, Finland, and Iceland due to reasons such as there being a strong alliance (Finland), neutrality (Sweden, Switzerland, Iceland), perceived resistance (England, southern France), and racial impurity (southern France).

The second category was to do with Lebensraum. Lebensraum was the concept of exterminating a local population of a land and replacing that population with Germanic or Germanised peoples. Countries that were considered fit for this process were mostly located in Eastern Europe. Countries such as Poland, Lithuania, Latvia, Estonia, Russia, Ukraine, Belarus, Czechoslovakia, Slovenia, Armenia, Azerbaijan, and Georgia were all considered for or placed under a Reichskommissariat, which was basically a colony for the local population to be exterminated or Germanized and for Germanic people from across the Reich to come in and settle. This land that these Reichskommissariat occupied were often rich in natural resources to exploit, such as oil in the Caucasus countries, or abundant in wide, open, fertile lands for Germanic people to farm and provide food for the ever-growing German powerhouse. In fact, Reichskommissariat were established in Germanic countries too. During WWII, four Reichskommissariat were properly established, these were Reichskommissariat Norwegen in Norway, Reichskommissariat Niederlande in the Netherlands, Reichskommissariat Ostland in Estonia, Latvia, Lithuania, and Belarus, and Reichskommissariat Ukraine in much of modern-day Ukraine. Others planned were the Reichskommissariat's of Belgium and Northern France, Don-Volga, Moscow, Caucasus, and Turkestan which were all viewed as viable for German colonisation. Poland is a notable absence from the Reichskommissariat list. This is because Poland was to be purified and directly annexed into the Reich, similar to how Austria and the Sudetenland were incorporated into Germany-proper. Except Poland had 35 million people that were not pure in Nazi eyes, and so it would take a little longer than Austria or the Sudetenland to be fully annexed.

All these countries, whether in the Germanic category, or the Lebensraum category were all considered as part of the Greater Germanic Reich. The Reich was to encompass most of Northern and Eastern Europe and would eventually be Germanised and fully populated with Germans who were racially pure. Hopefully this has been mostly accurate and has answered your question, I studied this a lot last year so hopefully I haven't left out too much.