System apps cannot be deleted.
Some can be disabled, which keeps them from running.
Android does a good job of making it difficult for a user to mess things up too bad, by making it difficult if not impossible to disable important apps.
Unfortunately, there are always some apps that you really don't need but have been deemed 'necessary' by the carrier/manufacturer.
Luckily, most of these tend to not cause much trouble.
The ones that I abhor above all are anything from Google.
Next on my hitlist are the 'helpful' ones from the manufacturer.
I really can't tell you what to eliminate, because what you and I want out of a device can vary greatly.
So, hit or miss (turn one off and see what happens) is the way to play this game, unless you can give specific details- such as what apps are using power, running in the background, able to do this or that, etc.
This can even be very different from device to device.
I was going to take and post a screenshot of what I have disabled, but forgot which device I was using.
This is my gaming device, so some Google junk is still active for games.
When I can, I will post another screenshot showing my daily driver, which has the most system junk shut off.
It probably won't fit in one screenshot, as I disabled everything that I could and still have a working device.
If you have opened developers options, near the beginning of options is 'Show running services' or similar.
You can at least see if any of the apps on your hitlist are running constantly.
From there you can go to your browser and look up specific apps to try to find out if you want that thing running or not.
If not, you can try to disable it.
(find the app)
If you cannot disable from there, go back one step and scroll to the bottom.
If the app still cannot be disabled, then you are most likely stuck with it.
So here is a screenshot of disabled apps on this device, and as I said I will post again from my other device.
As I check here, one screenshot is not near enough even for this device (used only on Wi-Fi for games and other such tripe).