Don’t Install macOS Big Sur on the same APFS container as older versions or you will no longer be able to receive system or service updates for your existing macOS version. And you'll see a "incompatible disk" warning.
https://www.macobserver.com/news/product-news/psa-big-sur-apfs-partition/
Further to this, you can still install Big Sur to another partition but only if your existing Mac's actual drive exceeds 256 GB, as that is the minimum size for an APFS container. Partitions cannot be created on APFS drives of 256 GB or less capacity – I tried with my 256 GB MacBook Pro & it wasn't permitted.
Therefore, if you want to try out Big Sur Beta on an existing Mac but only have 128 or 256 GB, it cannot be done on an internal drive, but you can install it to an external SSD formatted as APFS.