The page function has two parameters: header and footer. You can customize them by these two parameters.

A simple example is:

#set page(
  margin: (top: 40pt, bottom: 40pt),
  header: [
    #rect(
      fill: yellow,
      width: 100%,
      height: 100%
    )[
      Header in yellow block
    ]
  ],
  footer: [
    #rect(
      fill: red,
      width: 100%,
      height: 100%
    )[
      Footer in red block
    ]
  ]
)

#lorem(1000)

Here is the example in web app: https://typst.app/docs/reference/layout/page/#parameters-footer