Sometimes I read a first person story in past tense and I'm amazed at how much the main character is able to remember in such detail about events that happen decades ago. I know some books uses tools to explain the level of accuracy in the narration like a diary, letters, documents, video or audio recordings. But some fiction books in first person never tells you anything about this, why is that? Am I meant to assume something or leave it a mystery?
In situations like that the writer is using a narrator who is describing events not from a perspective of after the story has ended, but of a perspective of immediately after it happens. When you think to yourself about actions you just made, or actions you just saw, it's mostly in past tense. "I threw the ball." Or "He ran." Etc. When describing objects, however, we use present tense. "The gun is against the wall." Or "His pants are grey." Having a narrator a few seconds in the future prevents the tense mixing that we naturally have in our minds that might be confusing or considered bad writing. Having the entire story as present tense is another way around it. It's just stylistic choice and preference
At first, it would appear that the choice between past and present tense is binary. But the past starts a second ago and extends back for "a very long time". The further back into the past, the more unreliable that the narrator is likely to be. If there are several narrators, it is likely that each narrator will have a different perspective on the story and will remember different details of what did or did not happen. That perspective will certainly change over time: what is important to the young woman is not so important to that same woman who now has a dozen grandchildren.
You as a writer of fiction have the freedom to select the point of view that best serves your story. It is more work, but you might want to tell a simple story from multiple POVs to get a handle on how each POV contributes to the tone and texture of the story.