Hi Giovanna,
welcome to the Apple (user-to-user) Community.
Re: the new computer connects to the router but not to the internet.
Congratulations on the new computer!
For Ventura:
Information you need to connect your Mac to the internet – Apple Support
How to: Set up a connection to the internet on Mac – Apple Support
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Then, Apple support has an article online that may help trouble-shoot if needed:
What to do: If your Mac isn't connecting to the internet over Wi-Fi
Also:
Solve internet connection problems on Mac – Apple Support
Re: "The migration seemed to leave some files not fully transferred (some dropbox application files), but I do not understand where they are and how they can be connected to these issues".
It's also possible to transfer content from a Time Machine backup
(Sorry I do not know how else to help with this issue):
More support if needed:
MacBook Pro Support have a "Get Support" link (online chat, phone, eMail etc) at the bottom of their website:
MacBook Pro - Official Apple Support
All the best :-)