3i Infrastructure builds a varied portfolio by investing in companies that own and manage infrastructure assets around the world, especially in Europe, North America, and Asia. The fund primarily invests in private, unlisted firms through equity shares and shareholder loans, and it also includes junior or mezzanine debt for infrastructure projects. Additionally, 3i Infrastructure may invest in publicly traded infrastructure companies and other specialized investment funds managed by its team or external managers, ensuring broad exposure to different segments of the global infrastructure sector.