How can I make a link that opens a certain sheet or tab within a Google Sheet?
Use the 'gid' key in the link.
For example:
https://docs.google.com/spreadsheets/d/12E54t8oj9K9MDNILOXTwo5CkQiOlaozFP43lCObgB8k/edit?pref=2&pli=1#gid=1526123940
The used key is a random one used for an example. This is also true for the 'gid' key.
You can find the 'gid' number in the addressbar of your browser when the sheet you want to link to is selected. Use gid=0 for the first sheet within the Google Sheet