Not displaying iframe map result?

Question

JavaScript

https://www.openstreetmap.org/history#map=13/43.22... admin panel for working with maps
https://github.com/mokrayaGISka/Leaflet-starter-lesson one of the examples of working with openstreetmap and Leaflet, this document does not have an examples section and when you enter it you need to go to objects and when you go in you can see large.html and large.js
https://leafletjs.com/examples/zoom-levels/ and in Leaflet there is a map layers section and
https:/ /leafletjs.com/reference-1.6.0.html#polygon the documentation has a vector polygon

and the question is that I can't connect all this to my project, at the moment my project consists of a menu and a buttstrap 4 grid, and some small functions that work on several js files, I pull the iframe from the openstreetmap site and insert it into my project , it doesn't work and I can't understand why this is the first question, the second question is that I watched one video of Leaflet and openstreetmap slashing that they should work synchronously display objects on the map in the form of a circle with numbers and when approaching they should be divided into areas and in the end, display houses with addresses, but again, the fact that I don’t have a map, I don’t have Leaflet to display these objects on my project and naturally there are layers of tails, I have already searched a lot of articles about tails and did not find what I need, who knows, be sure to write,and I don’t understand why the Leaflet polygon is not displayed on my page, I copy and paste the code and it doesn’t work, I already directly connected it and created the js file this way and still I can’t display this is the third question ???

in short
, 1.iframe is not displayed
2.Leaflet and openstreetmap rounds are not displayed
3.polygon is also not displayed

who knows and faced this please help, I'm new to this, and I felt like getting a job in a company, and they gave me 4 weeks to prove myself, and 2 weeks have already passed, now it’s 3 weeks and I can’t solve this all common problem, thanks in advance to everyone !!!