Theme elements aren't responsive for tablet and phone

Hello, My website looks great viewed from a desktop/laptop browser but when viewed on a tablet or phone (or via customiser for different screen sizes) the design does not re-size the page elements to fit. URL: sloaneranger.com.au - Section Titles are cut off when scrolling down. - On mobile the header image isn't centred. - Site Logo isn't resized and takes up half the screen. - Making the header menu not sticky allows the site to be viewed better via mobile. - When content doesn't fit across the screen in a column, the subsequent columns go below the first column but they aren't aligned to each other. Why can't the contents of the columns shrink to fit across the screen? - When above happens the columns don't align to the centre of the screen. Some images are attached below: