It is really quite simple but not always evident
So, for babylon.js <script> tag HTML page with buttons have a look at this page - Mu-so 2nd Generation 3D
There are buttons to change textures, stop camera rotation etc.
If you view source of this page - it is well commented and should be clear to understand.
It was my first page with Babylon more than 4 years ago, by the way…
Thank You all - Pryme8 , M18 and labris very much for quick and right on the spot responses → to sum up, my problem was caused by the fact that the object I tried to access was defined as local (using const) and not global (using var, such as var sphere; and define it outside of any block/function) - what a shame
Also thanks for other useful info and links!!!