Some journals require copyright transfer, some don't. However, another issue is that respected scientific journals generally publish only "novel research" - i.

Some journals require copyright transfer, some don't. However, another issue is that respected scientific journals generally publish only "novel research" - i.e. no matter if copyright allows you to publish it, if you have published it somewhere then it disqualifies you from publishing that work a "good place" afterwards. So the "good journal" has to be the first publication and sometimes even if you retain copyrights they require an "embargo" that you won't publish it elsewhere (e.g. on your institutional site) for a year or two.