An unfinalised DVD has some control mechanism in the data that ensures
that only the machine that wrote it can play it back. A finalised VD
contains a specific folder and naming structure that matches what DVD
players look for.
I haven't used an iMac so can't go into specifics, but I have created a
DVD on a Windows PC and it played on a player.
You can't do it with operating system features, you have to use software
designed for the purpose.
I have used Nero in the past - there is a free version that does the
job if you choose the "Project" facility, but strangely it cannot play
back the DVD it creates unless you have the paid-for licence. Nero gives
you options of a DVD that can be added to (= not finalised) or one that
can't (= finalised). I have only asked for a finalised one.
I have now got Serif MoviePlus (there isn't a free one of that that
writes DVDs) because Serif allows me to edit the video and add titles
etc. Serif automatically produces a finalised DVD.
There are probably many other such products, but I have never looked
into them apart from PowerDirector which I looked at but didn't buy.
Whether any of these run on an iMac I don't know.
Jim