Exactly the way it did in the 20th century. I don't mind debating you but you're asking some elementary questions here. Do you not know how EI and Welfare came about ? I'm not saying this to call you out or insult you but you honestly seem to be asking questions about well-known points of economic history.
Not remotely comparable. You have to qualify for EI, which is self-funding, and welfare. A guaranteed basic income could simply be applied for. You can't be rejected because you don't feel like working and want to party. And when we combine this with the numbers of jobs likely to be eliminated by automation you're talking far greater numbers than welfare or EI, quite possibly a third of the population or eventually even half.
Your example is incorrect. Look on a macro level. If unemployment soars 10% due to automation then there is definitely a savings of more than those salaries. The idea is that on a macro level, efficiency savings are spread around the economy.
You're assuming that the automation would save 100% of the cost of the jobs eliminated, which is just not going to happen. The machines are going to cost money to buy, or more likely finance, and money to operate and maintain. You'll be lucky if the cost savings amount to 30% over salaries.