I've been through two reverse mergers in 2017, and it can be messy. Basically the fins of both companies must be audited, then sometimes there is an RS involved as well as a name change. All the assets of both companies are handed over to a third company during the merger, then the 3rd company is absorbed by the new company, at which time it is in business and tradeable. The fins can be problematic, as it must go back several years, although I can't say just how long.
One of the ones I was in exited the process pink stop - they had too many old debt issues to clear up. The new company took months to get to pink current. It is still faster and cheaper than an IPO, but it is really messy and most traders don't understand how it works.