Two new fields with a adjacent buttons are added to your city overview page. Here you can type an amount of infra or land to buy in all your cities. Purchases will be made once you press the button and accept the following confirmation modal. The field works like your normal infrastructure/land field. You can type a number, eg. 500 to buy 500 more infra/land in your cities. You can also type @ followed by a number, eg. @2500, which will do the math and make sure you end up with 2500 infrastructure/land in your cities.