What is a trading strategy?
A trading strategy is an ensemble of rules that will lead to a trading signal (buy or sell) in a systematic way. You can have an infinity of trading strategies. You can have strategies that are only working on certain assets, some others working on any assets without distinction, etc...
To build a trading strategy you can use any source of information or any indicator you want. For example, you can use the price of an asset. A simple trading strategy based only on the price of the asset would be: if the price of the assets drops more than 3% in one day I buy it and then I sell it when I've made a 5% profit on my position.
The problem with trading strategies comes from the fact that it requires a lot of technical expertise to gather the data you need for your trading strategy, then code your strategy to test if it is a good idea and finally if you are happy with it how to automatize everything; meaning how to execute automatically the orders coming from your strategy?