How to set headers and page numbers with alternating text on even and odd pages?

[quachpas]

@indoor_plant

where I can actually define header info

You can define the header struct in the header parameter of page. From your example right now, you have

context {
  if calc.odd(here().page()) {
    align(right, emph(hydra(1)))
  } else {
    align(left, emph(hydra(2)))
  }
}

which gives you alternating chapter/section header.
If you want a struct more like title | #, then you can use

context {
  let h = (
    counter(page).display(),
    h(1fr),
    hydra(1)
  )
  if calc.odd(here().page()) { h.join() } else { h.rev().join() }
}

which will give you alternating page number/ title.

@indoor_plant

also want the page numbers to start on the first chapter (page 7 in the file)

That’s as simple as using

#set page(numbering: none)

when you don’t want any numbering! Once your chapters start, you can re-enable numbering.

@indoor_plant

preferably be hidden on blank pages

I don’t think it’s possible to detect blank page? Except if you introduce one with a hard pagebreak(). In which case, you can set page numbering to none before breaking the page and re-enable it afterwards.