I can find the details on my phone's app, but not on my computer. On the classic I just clicked on "details", but I have not been able to find it on the modern.
If you are looking at the activity widget that is using list view, just click the activity name to load the activity page. If you don't have an activity widget, click the three horizontal bars icon on the left edge of the screen near the top, then click the word Activities. From there, click the activity name. There are tabs near the right side of the screen below the activity description. If Details isn't selected, you can click that tab.
