> If steroid mimicks testosterone. How is using steroid different than a man having naturally high testosterone level?
If your body perceives that there is too much testosterone, it stops producing its own testosterone. Your testicles will shrink. It may not take a very high dosage for this to start to happen. That won't be happening in a man with naturally high T who isn't supplementing.
By the way, how are your estrogen levels? Be sure that your doctor is keeping an eye on that. A number of things can cause your body to convert testosterone into estrogen, and you don't want high estrogen (for reasons of appearance as well as behavior). Conversely, there was at least one study some years ago which linked an increased risk of heart disease to men with both high testosterone and low estrogen. After supplementing with testosterone for about seven weeks or so, make sure your ratio of testosterone to estrogen is in check.
Another difference might be in how well you mimic the body's natural daily cycle of testosterone. A male's testosterone level is usually highest around 8am, declining throughout the day and at its lowest just before bedtime. (As it turns out, low levels of testosterone will help you fall asleep faster and sleep more soundly. Never supplement right before bedtime.) Those with low-T using patches, gels, and creams will generally apply them each morning and that does a great job of replicating the natural cycle. Additionally, their testosterone peak and lows are going to be more stable from day-to-day. That's exactly how it should be for a man with naturally high testosterone.
My understanding is that injections will give you a quick initial peak that will steadily decline (and when you're about ready to take your next injection, your testosterone levels may have fallen even further below what you started with). I don't believe that injections will preserve the natural daily cycle of testosterone (high in the morning, low at bedtime). I'm also wondering if the ratio between testosterone and estrogen remains stable throughout the peak and the decline. (My strong assumption: no.) If you're injecting, you'll want to research that and/or check with an expert. If you're already injecting, find an outside resource to confirm what the risks are and what your level of concern should be.
You didn't sound like you're cycling on/off your testosterone. But if you were, that would be another huge difference between yourself and a naturally high-T male.
I'm sure you're already aware that in both cases, high testosterone can have other unwanted effects like back hair, baldness / receding hairline, increased risk (or growth) of prostate cancer, increased anger, etc. You're likely to see at least one or more side-effect, especially when matching the level of a high-T male. Do not rely entirely on your own observations and opinions when monitoring for behavioral changes. Your best choice will be to rely on someone who you spend time with on a regular basis.
You seem to be aware that supplementation for low-T by a primary care physician is going to be substantially different than supplementing for bodybuilding. If you're going to a male health-and-wellness facility which intentionally tries to bring you to a high-T level, the advice which best applies to you is going to be somewhere in the middle of the other two groups. For more answers, you might want to find yourself a good subreddit. You'd be surprised by some of the high-quality answers you're going to be able to get over there. Still, I hope that all of this helps you in some way.
Disclaimer: I and most people here are not medical doctors, but I'm sure you knew that.