Reaper might be the best DAW that is “free.”
Technically there is a freetrail of the software but it doesn’t force you to get a license; the application is fully functional after the trial runs out. It runs on MAC and Windows.
You might need an software synth.
OpenShot is good free and open source software that runs on MAC and Windows for video editing. It is simple but the UI might be a bit ugly; it is as simple as iMovie in my opinion.
I personal use Logic Pro X(which costs $199) but there is a trial version. If you want to use the trial version forever, just switch the clock on your Mac to when you downloaded it, but some websites won’t work if you set your clock too far behind time. It comes with a softsynth and some drum samples. You have to tune your samples to the key of the song.