Briefly, t2g.txt contains the transcripts-to-gene mappings, cdna_t2c.txt contains all the cDNA (spliced) transcripts, and intron_t2c.txt contains all the "intronic" (i.e. unspliced) transcripts.
nucleus is used for single-nucleus data while lamanno is used for RNA velocity. There are subtle differences between the two workflows (e.g. for nucleus, the spliced+unspliced matrices are added up while for RNA velocity, separate matrices are generated that can be fed directly into the velocyto workflow).