Many researchers publish their research results through a publishing house with whom they enter into a contract. It often states in the contract that the publisher assumes all the rights to an article or a book. These are exclusive arrangements in which the author disclaims all economic rights. Your moral rights can, on the other hand, never be transferred.
If there is an exclusive agreement, the writer must ask the publisher for permission to publish an article on their own or their institution's website. The same applies if the author wishes to publish an article as part of another work, with another publisher or as part of a PhD thesis.
A very large proportion of international journals currently make it possible to apply quickly for permission to republish your own articles. Most of them use a service called 'RightsLink', which is handled by the 'Copyright Clearance Center'.
To do this you need to create an account and then apply for permission.