Long-term solution: have a button to add a past session appear when we click on a past day on the calendar.

Current workaround:

Option 1: If that was a new kind of workout for you, click on "Empty workout session" (at the bottom of the workout page), then log everything you did yesterday and finish the Session. Once completed, open the session and click on the 3 dots (top-right corner) > Edit to change the date and duration to match what you did in reality.

Option 2: If the workout is similar to something you've done in the past, then find that similar workout in the calendar, click on "Start session" and log things as you did yesterday. The remaining steps are the same as in option 1.

Your experience, muscle fatigue, PRs, XP etc. directly depends on your workout sessions, so you can always edit them like that, and the rest of your data will instananeously adapt to your sessions.