'By default' could be expressed more explicit here. Plone 6 will use Volto by default if you create a new Plone 6 Site.
I realise the following is very difficult to plan or predict, but shouldn't we add more information and guidance for existing users of Plone 5 on migration?
AFAICS at this moment (december 2020) Plone 6.0 will not offer an 'out of the box' migration from Plone 5.2 to Plone 6 with Volto React frontend.
There are too many technical issues and edge cases to solve to pull this off. It doesn't mean its not possible, but it's not included or supported in the core Plone 6.0 distribution.
You can migrate from core Plone 5.2 to core Plone 6.0 Classic, but only if you have Plone 5.2 already running with Python 3.
With this migration from Plone 5.2 to Plone 6.0 the usual care has to taken with customisations and the use and compatibility of add'ons. Also because of Barceloneta LTS users/integrators should expect more work to update their custom themes as the default html templates in Plone 6 Classic have been changed to only contain Bootstrap 5 css classes.
[Edit: I propose to add this because of previous questions here like Volto on an existing site - #3 by fredvd]