Fact sheet from the National Library of Medicine on the differences.
Why have two platforms for Medline (PubMed and Ovid Medline)?
Searchers have personal preferences and find one or the other easier to use. Ovid Medline has more search functionality (proximity searching for example). PubMed can be marginally more up to date as data from publishers flows first to PubMed and from there out to other database providers.
Searching a variety of databases is required when doing research.
Although there may be considerable overlap between the coverage of different databases, each indexes unique titles and uses unique thesaurus terms to index the literature.
Online database tutorials are available.
NOTE that CINAHL is not available.
Collections such as those below may also be helpful.