If you specifically need the old iTunes interface for compatibility with older software or personal preference, some users use the third-party tool Retroactive macOS Big Sur - App Store
If you are looking for where your old iTunes content went: itunes macos big sur 117
If you just upgraded to Big Sur and miss iTunes, give the new system one week. Open Finder, plug in your phone, and explore. You will realize that nothing is actually gone – it has just moved to a more logical home. If you specifically need the old iTunes interface
If you are looking for the "iTunes experience" on Big Sur 11.7, here is where to find every feature and how to manage your media. 🎵 Where Did My Music Go? plug in your phone