Signature campaigns are are most common way for me to invest in Bitcoin, it is because you only need to invest your precious time and effort then get paid in Bitcoin. Other enthusiasts that has extra money they prefer buying Bitcoin. But for those who has skills to offer, they are paid for their services with Bitcoin as well. Since I have no technical skills and extra money, I do signature campaigns that let me earn Bitcoin in weekly basis.

Bounties and signature campaigns are different.

When talking about bounties, it usually about tokens and tasks on social media. They barely run bounties with signature campaigns as part of their bounties. If they have a signature campaign, its payment is in token too.

When talking about signature campaigns, in my opinion, it is about campaigns pay participants in Bitcoin or stable coins. Payment rates in those campaigns are better than in bounties. However requirements from managers will be higher and you must be competitive than other participants, with good post quality history to be chosen.

You can find jobs, work and get bitcoin or altcoins then convert (sell) altcoins to have bitcoin.
Cryptocurrency job list
[Guide and Advice] New Here? How to Earn and Learn.

Common ways to invest in Bitcoin, really?are there common ways we must actually follow, maybe you should begin with this by learning first about bitcoin, then learn to earn it or exchange your fiat for it and hold, how then should you hold, you can choose to be buying it or earning it gradually and investing on it, then choose the wallet to use and hold your bitcoin, try to understand the target you have on your investment and lay a good track record of Bitcoin price performance over time, the more you keep holding the more your chances increases in making profit on your investment, speculate thoroughly, check your market price performance every time and you're good to go.

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.

The common way of investing bitcoin is buy bitcoin in small quantity by accumulating them , this will really help you to afford to invest in bitcoin if your earning is not big to start big just as some people claim like it is compulsory to start bitcoin investment with huge amount of money. And one of the common way of investing in bitcoin their must be a source of income which money flows in, without this it is impossible to invest in bitcoin. It is not possible to invest income when their is no steady flow of money that one can always look unto .

You join a signature campaign and every week you get paid in bitcoins from here. Which is why you can use it as a Bitcoin investment medium, but it won't work for everyone. So I think the best way to invest in Bitcoin is to set aside some percentage of your monthly income for Bitcoin. And to continue this investment. Then it will follow DCA which will reduce your risk a lot. This is the investment plan and you must hold it long term for profit