Vielen Dank, Erich!
The compiler is creating the tusb.o object file:

And that object file comes from tusb.h and tusb.c (where tusb_init() is defined):

I saw your post about exclusion earlier and made sure that the exclusion check box was not checked.

and I looked at the alternative that you suggested and it seems to be on the default (assuming that means include in build)

all the best,
James