what's the difference between "at the end" & "in the end" and between "in time" & "on time"?

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"At the end" is used when recurring to something specific ( a movie, book, story. etc.)
"I didn't like what happened at the end of the book."
"The movie has a surprise twist at the end."
"In the end" is more abstract and means something like eventually or after all.
"The test was difficult, but in the end, I think I did OK."
"If we practice hard, we will be the champions in the end."
"In time" can also mean eventually.
"We all believe that, in time, we will find a cure for cancer."
It can also mean not late, within the scheduled time. "In time", in this sense, always refers to something you were able to do.
"I arrived in time to see the wedding."
"I was afraid that I would miss my plane, but i arrived at the airport just in time."
"On time" describes that an event was punctual.
"The bus is usually late, but today it arrived on time."
"Did the meeting start on time, or was it delayed?"
You can combine them.
"The bus arrived in time for me to get to work on time."