What does it mean when submission system says "under review" after submitting minor revisions?
In most of the article submission systems the "Under Review" status means your manuscript is with reviewers. Most probably it must be sent to the old reviewers but there is also a possibility that editor might invite a new reviewer. It happened to me, one of my article which went through 2 revisions, 1 major and 1 minor took 1 year, and it was rejected because editor invited a 4th reviewer who raised few concerns. 
Best of luck for your article.