You’ve heard the story about the famous early 20th Century astronomer who received a peremptory telegram from William Randolph Hearst: “Is there life on Mars? Cable 1000 words soonest.” And his response was prompt: “Nobody knows. Nobody knows.” for 500 repetitions.
The Celts occupied a large area centered on France at the time we encounter them in history. The Helvetii were in the upper Rhone valley and gave their name to Switzerland (Helvetia). There were Celts in much of Spain, western Germany, northwestern Italy, and the British Isles. The incursions into the Roman and Greek lands were by roaming bands.
The Celts speak (and spoke) a group of Indo-European languages most closely related to the Italic languages from which Latin and the Romance languages sprung – the other Italic tongues were overrun by Latin speakers in early Roman times, and only small samples of their languages are extant.
There is a very strong hypothesis that ties them to the Beaker Folk, the archaeologically-known group that lived in the same area a few hundred years before the Celts become known to history. This is not proven but the evidence linking them is fairly strong, so I’m told.
Like nearly everybody else European, the Celts seem to have originated in Eastern Europe, more or less in the Ukraine (geographically, not necessarily in the specific modern nation), and to have migrated west. This is the most likely site for the Proto-Indo-European peoples which expanded from roughly that area. And the PIE folks would have given rise to the Celts, along with the Italic peoples, the Germanic peoples, the Balts and Slavs, etc.
Worth noting is that while there are strong Celtic admixtures in the genetics and linguistics of the French, Spanish, etc., nearly* the only place in which they survived as a distinct people preserving their own languages past Roman times is the British Isles – Brittany on the Armorica Peninsula of France being a derivative of Insular Celtic as opposed to Continental Celtic.
*“Nearly” because there are fragmentary records of post-500 AD Celtic cultures and languages here and there which are now extinct with no real surviving remnants. The Galatians whom Paul evangelized and wrote a letter to, for example, are a group of Celts who ravaged part of Anatolia and then settled down and built their own small colonial area in what is now east central Turkey. They’re on record as still preserving some Celtic traditions and language in about 800 AD, but apparently not a whole lot later than that.