Your question is really too general and too broad.
Some data/workloads can be straightforward migrated without 3rd party tools, other may require 3rd party tools, some can be migrated partially (with or without 3rd party tools), some not at all. The M365 stack is really too wide, Mail, files, SharePoint, Power Platform, Viva, Teams, forms, sway, bookings, shifts etc.
Even licensing is wide. What kind of license? Online, CSP, Enterprise Agreements, monthly, annual, corporate, subsidiary etc.
If you are a M365 customer and have something specific in mind, perhaps you can give the details then others can chip in from their experience and knowledge.