Is it possible to update to OS BigSur from High Sierra on a 2015 IMac

Yes, it is possible to upgrade to Big Sur – or as den.thed indicated, to Monterey.

Yes, it may affect connected devices and existing applications. If you upgrade to Catalina, Big Sur, or Monterey:

1. 32-bit applications will stop working. You might want to identify which 32-bit applications you're using now and decide what to do about upgrading or replacing them.
2. iTunes will go away. Starting in Catalina, Apple divided up most of the functions of iTunes between several new applications (Music, TV, Podcasts) and the Finder.

This is not a comprehensive list of all possible changes or incompatibilities.

What happened to iTunes? - Apple Support

A 2015 iMac can be upgraded to macOS Monterey.

see > Identify your iMac model - Apple Support

To upgrade from High Sierra to Mojave, Catalina or Monterey,

see > How to download and install macOS - Apple Support

Just to add, Catalina and later are 64bit only so older 32bit apps would need to be updated or replaced. You can check which you might have with Go64