on PC— when you open an activity from the past and then go back using the Back button, the app doesn’t return to the place where that activity was located. Instead, it takes you back to the current week’s screen. Are there any settings that would make the Back button on the mouse return to the screen where that activity was located?
No, that’s not configurable. Intervals is browser based. When scrolling down the calendar, the url remains the same. And when using the back button, you go back to that same url.
Something that might facilitate what you’re trying to do, is running Intervals in 2 or more Tabs/Windows. Scroll the calendar in one, then use right click and ‘Open in new Tab/Window’
It’s really annoying. Imagine browsing this forum and every time you click the Back button on your mouse, it takes you all the way back to the top of the page. Super convenient, right? 
???, my fault??
Simple, dont scroll the calendar! 
Use the time settings and the URL will change, as such you if you press Back it will go to the correct screen.
Alas your analogy is wrong. If you scroll on the Forum and open a Forum Post you would expect hitting Back to get you to the Forum at the same scroll position you had. Which is unrealistic.
HTH!
This process in web development is called scroll restoration. - The main name for the browser mechanism that ensures when a user clicks the back button, the page scrolls precisely to the position where they left off. And there are appropriate settings that the programmer can use history.scrollRestoration, History Stack / Session History - The browser’s memory that stores visited URLs along with the saved coordinates of the page scroll - scrolling on the x and y, this mechanism is fully applied to this forum, what is unrealistic for you is actually applied here, it’s hard not to notice it, The developer has all the tools needed to make this work, even in an SPA. They just need to actually use them