When one company takes control of another, this is known as an acquisition. Sometimes after an acquisition both companies will remain separate entities and at other times they will merge to form one larger company.
For example…
Facebook is a company famous for making acquisitions, including Instagram for $1 billion and WhatsApp for $21 billion. Its many acquisitions have been strategic tactics to grow the services it is able to offer its many users without having to spend time and development dollars perfecting these services in-house.