Some people think John 23rd was valid, but most don't.

Originally, many sedevacantists used the argument that those people lost their office due to public heresy. But in recent years Fr. Cekada has been arguing that their elections themselves were invalid due to their heresy.

Canon law requires that a pope be Catholic in order for the election to be valid. But it's obvious that these people were not Catholic before their election. (The reason it's obvious is that, if you do think they were Catholic before their election, then you are saying that election to the papacy turned them from Catholics into raging heretics, which is impossible and absurd).

If the entire college of Cardinals voted unanimously to elect the Dalai Lama as the next pope, it would be invalid because the Dalai Lama is not Catholic.

Francis was never pope because he was not Catholic at the time of his election.

Would you have any further information on this?

I had to do some digging and found this interesting book by a CMRI priest.

https://archive.org/details/WhatHasHappenedToTheCathoRadeckiFr.FranciscoCMRI5025

June 1, 1914, Angelo Roncalli visited Rome with the Seminary rector of Bergamo. Cardinal Giuseppe De Lai, who was the secretary of the Consistorial Congregation, suspected Roncalli of being a Modernist, began a background check on him and kept his activities under close watch.

1921, One of Roncalli’s closest friends in the seminary was Ernesto Buonaiuti who assisted Fr. Roncalli at his first Mass in 1904. Buonaiuti was excommunicated in 1921 and again in 1924. He was declared vitandus in 1926. (The term vitandus means that the pope personally expelled him from the Catholic Church.)

1924, Roncalli was given a teaching position at the Lateran University until he was suspected of Modernism. He was thereupon relieved of his post. For the remainder of the year, Roncalli fulfilled tasks of minor importance at the Vatican but was closely watched. The Modernist label given to Roncalli, carried with him for the remainder of his life.

1958, “Shortly after he had become Pope. . . Roncalli visited a certain Congregation’s office and asked for his personal file. He said that he wanted to find out why a routine promotion had been so long delayed. Looking back to those early years he found against his name the damning words: ‘suspected of Modernism’.