Depending on the intensity of your training, we can assume that most people need to go sympathetic on at least some level in order to have a productive training session. Time of day, nutrient timing, stress levels, quality of sleep, etc. will all play a role on our ability to go into a sympathetic state for training. On the flip side, when the training is over, we want to be able to move out of that sympathetic state and enter a more parasympathetic state. We need to rebuild/model tissue, adapt to the training stimulus, rest and digest, and recover before our next session. This doesn't necessarily mean we lie around and do nothing, it could be more active recovery or doing activities that bring relaxation and a more peaceful state. This will definitely look different for each and every person.
How do we know whether or not we are balancing between these two states? How do we know when we are primed to train or in need of rest? There are tools like Heart Rate Variability, Recovery and Resting HR, and other Biomarkers. I think the missing ingredient for most people and the first place to start is to just be present, being in tune with your body, and recognizing the signals. This is hard for most of us and it is a skill that needs to be learned, maybe our signals have been muted or crossed for so long. Sometimes we mindlessly follow the program, check the boxes, hit the prescribed reps, sets, percentages, etc. and give no conscious thought to what we are doing. When it comes to autoregulation, most of us need to start with being present. Most of us need to be present in our training and learn to be in tune with our bodies. We need to stop doing things that mess up the signals and learn to recognize the signals. Doing so is the first step to longevity in our training and long term progress towards our goals. There are plenty of resources out there if you want to go deeper into the rabbit hole.