For a digital existence to be possible, you either would need an isolated brain, for which the body has been discarded, or a fully uploaded mind, in which we left the organic substrate altogether in favor of a synthetic one. A virtual world in which your body is kept around and taken care by a support machine does not seem feasible to me, as there are too many points of failure, from diseases, to aging, and muscular atrophy. A single isolated brain connected to an artificial support, on the other hand, seems far more feasible.

For there to be an UBI, there needs to be scarcity of basic needs. In all likelihood, there won't be anything truly essential and scarse that one or a group of humans can offer to others in exchange for money, considering all brains will already be able to generate whatever they want and imagine on their own system. However, assuming there still is a differential in intelligence, the most capable minds will congregate to advance the technological dependencies that everyone relies on, such as the world generators, brain support machines, artificial reproduction pipelines, exowombs, energy supplies, longevity treatments, molecule builders, etc. They will be compensated by their efforts via having access to the latest technologies first, whereas everyone else will simply wait until it is made available for them. Only a minority of highly gifted brains will participate in the economy and be producers of technology, whereas everyone else will simply be consumers.

This is assuming we achieve brain isolation after AGI, but before ASI, which is not necessarily going to be the case. If we reach ASI first, then there will be no human producers in the first place and, if mind upload is possible, it will be readily achievable by the ASI. An independent and well aligned ASI will likely make the whole notion of a market economy obsolete. Everyone will simply live in their own worlds or cross over to other people’s worlds and public realms. Some will fully retreat and never interact with other humans again, whereas others will constantly congregate with their previous family and friends.

I don’t know much about neurobiology, but I believe there are limitations to how much pleasure an individual can induce before reaching several forms of neurological damage and intrinsic limits. So it might be the case that simply bombarding yourself with pleasure chemicals is not going to work, and a more natural distribution of positive and negative emotion, resembling our present reality, will still be necessary for self-preservation. Even though isolated brains won’t be able to have endless chemically induced orgasms and serotonin overloads, the lows of poverty, disease, anxiety and depression will just cease to exist.