Other investors only know about they trading platform and even doesn't think about other questions how investments are traded. Te thing is that there are two types of markets: centralized markets as stocks exchanges and non centralized markets that also are called as OTC markets.
OTC markets are those markets that create themselves naturally and if you would sell some financial instrument to your friend and it will be a transaction of OTC (over the counter) market. The name of OTC market is not very popular but in reality there are trillions of deals made in OTC market and it has very important share in OTC markets and that is natural way to develop even more in the future because of increasing computerization.
For example acquisition of one big company by another is also an OTC market deals, especially if that acquisition is by privately held company and the target of the acquisition is also private company. Let's say one guy (shareholder) meets another guy (investor) and the last one buys the shares from the first one. That's how financial markets does the job.
Of course if acquisitions are made for listed companies such acquisitions may get more complicated to explain. However acquisitions in M&A and OTC markets are very closely related and may be interesting to see the developments of these fields in the future.