Could anyone migrate to France if they chose to? Was proof of employment / means to fund oneself required? Did this vary depending on the country you were coming from? How much changed from pre-WWI to post-WWII?
(Inspired by Julia Child writing of her sister coming to France to stay with her in Paris and looking for a job without knowing much French / having any employment experience in ~1946. No mention was made of any formal process to do so - was there one?)
After WWII, Europe was in rough shape. Besides many cities damaged from bombs, there were also thousands upon thousands of displaced people all across Europe. These people were Holocaust Survivors, released prisoners of war, people who lost their homes, and the list goes on. Economies were also in rough shaped as many former Nazi-occupied countries regained their independence. Even non-occupied countries were in rough shape, because all of the countries felt the effects of the war. There were also millions who were dead.
After the war, across Europe, displaced persons camps began to pop up. Many were near former concentration camps to help survivors recover. There were also displaced persons camps that took in anyone who could not get home or did not have a home to go to. There were also many people fleeing West because they did not want to live in the Soviet Union. The influx of people led to mass immigration across Europe and beyond. Those fleeing their home and those who did not have a home to go to needed a new home. This is where the post-WWII immigration begins. France was a popular choice by many people.
Even prior to the war, France attracted many immigrants. It was a growing country with opportunity. It was also welcoming of refugees from other parts of the world. France also attracted immigrants from its colonies. Prior to WWI, it was fairly easy for other Europeans to move between countries and immigrate, espescially when countries shared a border. Coming from overseas was harder, like an immigrant going from France to the United States or the other way around.
After deadly wars, there was a need for labor and immigrants often filled those gaps. After WWII, the situation was very much the same. France needed to rebuild and workers were needed. With many refugees across Europe, many chose to move to France to fill the work shortage. Immigrants found work mostly in manual labor jobs like mining, construction, farming, in factories. These jobs did not require knowledge of the French language. This applied evenly to men and women. Men found more jobs in agricultire, mining, and construction. Women found more jobs in factories, especially where clothing was made. Many of these skills were known by the immigrants prior to coming to France.
The French government was also eager to get more immigrants to help rebuild. France was present worldwide. It attracted immigrants from other European countries, from its colonies such as Algeria, to as far away as Vietnam. If one had the skills that France wanted during post WWII times, then they were welcomed to France. No proof of funds was needed. With the right skills, one could easily find a job.
Others were sponsored by people and then brought to France. This was common among people displaced after the war. Holocaust survivors who had no family or home to return to often took the job offer so they could slowly rebuild their life. Many viewed the post war jobs as a way to move on and find a new purpose in life. They also used them as a way to save money before immigrating further, like to the United States. After WWII, immigration did change, especially after countries were rebuilt. It did become more strict and there was a growing push for more regulation in France, especially by the 1960's and 1970's and the EU coming into existance.
Sources: Labor Immigration in France, 1946 - 1965 by James R. McDonald
Immigration and Integration in France, 1945 -1974 by Matthias Grenon, Boston University
France - Immigration by Britannica