IDs kept: article, category and tag IDs and aliases stay the same, so ID links keep working.
Passwords kept: modern (bcrypt) password hashes are copied, so users log in with their old passwords.
Groups and permissions: core groups are matched by name, and permissions and access levels are remapped.
Custom fields and tags: fields, field groups, values and tag links.
Check first: a free check shows counts and warnings. Nothing changes until you click Migrate.
Safe: the migration runs in one transaction and rolls back on any error.
Admin screen or CLI: php cli/joomla.php bridge35:migrate for big sites.

Not moved: menus, modules, templates and third-party extensions (K2, VirtueMart, Community Builder). Install those fresh on Joomla 5.

Requires a fresh Joomla 5 site and read access to the Joomla 3 database. Tested on Joomla 5.4.10, PHP 8.3 and MariaDB 11.4.