RSUs are immediately taxable upon vesting. Companies often cover this, but it's at the discretion of the company.

I've only ever heard of companies offering to withhold enough shares to cover the tax obligation. Is that what you mean, or do you really mean giving the employee an additional cash bonus to cover the taxes?

They're taxable on delivery. For public companies that's almost always at the same time as vesting, but for private companies that's generally at the time of an exit.