What are the most common ways to invest in bitcoin now?
You asked about investment methods, but you did not ask about buying Bitcoin. Investment methods vary depending on your experience, your country, and access to surplus funds, and the simplest and most famous method is Buy and Hold for a period of time, at least 4 years.
other popular methods are such as DCA, Buy Low, Sell High, and some unconventional methods from Buy Bitcoin Stocks such as Greyscale’s Bitcoin Investment Trust (GBTC). Therefore, without customization, you will find that the answer may give thousands of common ways.