I have read online food takes about 7 hours to digest, if this is true, what is the point of eating carbs shortly before a workout, or even consuming pre workout?
Different types of foods take different amounts of time to digest. Fats, proteins, and complex carbs can take 4-6 hours. Simple carbs are a different story, though. Something like gummy bears, white bread, etc will digest much more quickly
The concept of pre, post, or intra-workout nutrition is primarily a marketing ploy to the average gym goer. Most of your nutritional needs for workouts are based on the total days nutrition (or the previous day). Unless you are doing long workouts 90 minutes plus. There some evidence to suggest that there is a small benefit to post workout carbs for glycogen stores but again, probably not going to move the needle unless you are working out for long periods of time or doing multiple workouts a day. The “post workout protein shake” is a classic example of nutrient timing perpetuated by the supplement industry that doesn’t really have any scientific data but has been adopted by the masses, cause gains bro. Of course I am not a dietician, but I spent way to much time worrying about this at one point haha.
The point of eating carbs shortly before a workout is to have enough free glucose to power your workout.
There’s a downside, however - if you have a lot of free glucose and you exercise you will mostly burn glucose during the workout and not burn much fat. You may also get hungry and then eat back the carbs that you burned.
For people who want to lose weight, this isn’t great. What they would like to do is burn fat calories during their exercise and not eat those calories back. Metabolically, the best way to do that is to work out at low intensities (zone 2 ish) in a state without lots of free glucose - fasted is a common approach.
To answer your specific question, if you eat something carby, blood glucose will start to go up in about 15 minutes