Some of these e.g. L1 are from the retrotransposon itself, others are from any cellular gene. Normally RT not found in cells, but apparently it is occasionally active. Exons are all sequences in mRNA, whether translated or not. Usually some sequences at 5' and 3' ends not translated. Found in 1977 by R-looping.
Histone genes have no introns.
Introns found in same place in all related genes e.g. globins. Even insect globins. Size of introns 100-100,000 bp Total number of genes is unknown. YOu miss ones with no major effect. About 1 gene per band in Drosophila, so about 5000 genes.