MC is [transmigrated/reincarnated/hit with a meteor/picked up an object]. MC is most of the time [trash/is bullied/strong to weak/lost talent].
The system's format may show hp/sp/mp/vit/str/def/int/wis/luck/dexterity/agility/skills(passive and active) /titles/job/class/etc. The system may come with an A.I. Now the author decided on how smart the A.I. Should be, could it actively help/harm the mc, or is it just a passive system that reports what happens and what the mc does. This system may change at any time from active to passive based on how the story proceeds.
Now, the story actually begins with the system being a big/small help to the MC. The help the system provides could fluctuate throughout the story, the system may even become dormant for a long time. The system could provide a companion/pet/couple page.
Now time skip... Where are we... I see so now we're halfway through the story. Now the Author can choose to start displaying hints of how the system came about if the author did not state it at the beginning already. Could the system be from another dimension? From a failed experiment from the past life? A bored god messing with people? The author can also decide on the neutrality of the system. Did the system purposely hide a few problems with nefarious intent? Or is the system genuinely there to help the MC. OK. Now let's skip to closer to the end.
So now that we're closer to the end, the author can make another decision on how to deal with the system. Want to make the A.I. come to life as a friend/companion/couple? Release the god that was acting as the system? Or is the system evil and the MC rips it out himself and throws it in the next dimension. Up to the author this should be the end of the system as most stories that reach this state would explain everything instead of leaving it blank... Totally not throwing shade at some Authors.... Anyway, this should be the end of this chapter.