• link https://alpinerally.org.au/category/posts/galleries/  
  presents all posts with category “galleries”
• link https://alpinerally.org.au/tag/alp09/
  presents all posts with tag “alp09”
• link https://alpinerally.org.au/?cat=77&tag=alp09
  presents posts with category 77 (heritage) AND tag alp09
• link https://alpinerally.org.au/?cat=2&tag=alp21,alp19
  presents posts with category 2 (blog) AND tag alp21 or alp19
• link https://alpinerally.org.au/?cat=2&tag=alp21+alpine-team  
  presents posts with category 2 (blog) AND tags alp21 AND alpine-team   

To find the cat number go into the WP Dashboard, go to Posts > Categories and edit a category. The ID of that category can be found in the middle of the edit page URL…
eg  https://alpinerally.org.au/wp-admin/term.php?taxonomy=category&tag_ID=2&post_type=post&wp_http_referer=%2Fwp-admin%2Fedit-tags.php%3Ftaxonomy%3Dcategory
in this case, you can see ID=2 in the URL. That’s for category = blog. 

These link will gather all posts that satisfy the parameters and present them in a posts archive template – the archive-default template or a specific template for a category if there is one. Eg, you could have a different archive template form for heritage post or news posts, with all the others being handled by archive.default.