A very interesting area. I have heard from both @WHO and @NICU_doc_salone that what's vital isn't punitive enforcement but SUPPORT for quarantine.
The NZ and Aus legislation on mandatory self-isolation and quarantine are here...
https://legislation.govt.nz/regulation/public/2020/0241/latest/LMS401667.html#d10374790e1370
https://www.health.nsw.gov.au/Infectious/covid-19/Pages/public-health-orders.aspx#isolation https://twitter.com/joyofhazel/status/1353281231362916352

If you only have a few infections, you can really put resources into making sure they don't spread further.
In Australia, if you have to self-isolate you are handed an official order that explains exactly what you can and can't do... https://www.health.nsw.gov.au/Infectious/factsheets/Pages/self-isolation-covid-and-close.aspx

Getting an official order on paper in itself is enough to get compliance from the vast majority of people, @NICU_doc_salone told me.
The order also notes that if you break the rules, the maximum penalty is a $11,000+ fine and six months in prison... https://www.health.nsw.gov.au/Infectious/factsheets/Pages/self-isolation-covid-and-close.aspx"

But more important than the punitive threat is the support. The health system is capacious enough and the cases few enough to:
- have doctors visit isolated people at home
- bring them groceries
- check up on them every day
etc etc...

The @WHO emphasises that just telling people to isolate is not enough, because studies show a large % of people don't do it -- for understandable reasons! They have to buy food, they fear getting fired, childcare reasons, etc. You have to support them. https://www.theguardian.com/world/2020/sep/11/less-than-20-of-people-in-england-self-isolate-fully-sage-says

Europe and the US are at a disadvantage here because 1. they have thin public health systems to begin with and 2. they chose to 'keep hospitals from being overwhelmed' instead of hunting out and isolating all cases. So now there are an overwhelming number of cases to manage.

Each lockdown provides a chance to change that though. It's down to policy choices. The worry is that if we keep choosing not to get a grip on this we'll keep getting new mutations and buy ourselves a permanent pandemic.