You have asked a question about the present narrative simple tense. I would begin by renaming it. We are talking about the present tense. Don't make life complicated. It's not about many different tenses, it's about one tense and learning the range of contexts where it might be used.
Second point. This is not a grammatical issue, it's a stylistic and literary issue. There will be a grammatical component to your writing, in that if you use the present tense in any given sentence, it will need to follow the rules of tense construction for that verb. But that's not what you're asking. You want to know if you should choose the present tense in the first place. That's a question of style.
Which leads to my third point. You ask when you should switch to present tense in a story. This implies that you have to use the present tense in some contexts. I would say that this is wrong. You could spend the rest of your life using the past tense to tell stories from the past, and you would be doing fine. Using the present tense is not a requirement, it's an option. Think of it as a tool in your tool box, one of many, and gradually learn how you could use it (even though you don't need to.)
I would give you two suggestions to help you develop in this area. The first suggestion is one you are already doing. Read and listen to several examples, such as the YouTube videos, and consider how the present tense is being used. What parts of the story were told in the present tense format? What difference did it make to the feel of the story telling? (For example, using the present tense can give a feeling of immediacy to the reader. The story is told as if it's happening now, not 25 years ago. We feel like we are right there with the characters as the story unfolds.)
The second suggestion is to think of the narrative structure as a series of brackets, which can guide your choice of tenses as you write. To use your example, I could lay it out like this:
I'm gonna tell you about my grandpa. (Future tense, because at the end of this sentence everything you say about your grandfather is still ahead of us.)
He's a retired teacher who is excellent at Maths. (Present tense, because you're talking about who he is now. But suppose your grandfather has died; then you might say he was a retired teacher. Remember, no rules of grammar here, simply choices of tense to say what you want to say.)
I can still vividly remember those days when he was younger, he would take me (now you're switching to the past tense because it's a past memory. Perfectly reasonable.) to the playground near my house to teach me how to play football, all while carrying a briefcase of exam papers and documents to work whenever he had a moment.
On one occasion I saw him working in his garden. My mum had just bought me a compass, so I go up to him and show him. He smiles and says how special it must be. He asks me if I know how it works, and I'm glad because I don't know, and I'm hoping he will show me. I know how clever he is with such things. (If you're observant you will see that I have changed from past tense to present tense during the course of this story. This is because I'm now telling a particular story as an illustration of grandpa's general willingness to help and to teach me. The general habits are past tense, but the switch to the present tense for this specific story draws the reader in. Then I might return to the past tense at the end of this segment.)
In summary, no rules that you have to follow, just building experience and gradually learning how to use each tense in ways that say what you want to say. Good luck!
