1) I have seen everything today
2) I saw everything today
How is the first line different from the 2nd please explain?
You can say "I saw everything today" if you are thinking of a specific moment earlier in the day that has finished, indeed if the day has ended, or you saw it in the morning or afternoon and it is now early evening for example. "I have seen everything today" could mean you have seen whatever "everything" may be today and the day hasn't finished yet. The experience is still fresh in your mind. It could also mean some kind of astonishment. It is an expression: "I think I have seen everything", meaning you have seen something you have never seen before and you find it extraordinary or weird.
Besides the fact that (have seen) is in the present perfect tense while (saw) is in the past simple tense, the difference can be considered as the following:
1) I have seen everything today.
You saw everything that happened "today" with one of the following possibilities:
1- You saw everything that happened today till the very last moment of the day.
2- What you saw today has some connection to the present time of your speech. It may be an effect, impact, consequence ...etc
2) I saw everything today.
You saw everything that happened "today" considering the day has ended.