I want to know how actually (and on the basis of what) visiting researcher (for PhD students) or visiting Ass. Professor (for lecturers) opportunities are offered worldwide (especially for USA/Canada) ?
Thanks !
For the European countries I've seen the answer would be similar:
Visiting researcher is not really a type of (funded) position, but a status. PhD students might do a research visit as part of their doctoral studies while still being paid by their home institution, sometimes supported by a travel grant to cover travel and housing costs. There are also post-doc grants providing 1- or 2-year funding that are implemented by having the involved institution host the postdoc as a visiting researcher. Finally, a professor on a sabbatical might visit another institution for a couple of months and in the course of that take on the visiting researcher status.
Fixed-term lecturer or teacher positions are created by the department as a regular type of position (the term "visiting assistant professor" is usually not used). They usually have to be publicly announced and advertised, although the effort for advertisement is usually smaller than for a tenure-track or tenured position.