My battery drain so fast

All rechargeable batteries are consumable components that become less effective as they "chemically age" (note that is not only just the passage of time; there are different factors, the main one is the number of charge cycles).

In order to slow down that inevitable process I would suggest you to:

• If possible, keep the battery level of your iPhone between 40% and 80% (charging below/above those values is more "stressful" for the battery)
• If your battery level goes down to 20%, turn on "low power mode" (reduces display brightness, optimizes device performance, minimizes system animations, etc.) to avoid to completely drain your battery (0%). You should not let your battery drain to zero, because it will damage your lithium-ion battery. If that happens, charge your iPhone as soon as possible!
• You can fast charge your iPhone (8 and later models) when you use an Apple USB-C to Lightning cable and a compatible adapter (18W* or higher). In terms of battery level, this means up to 50% battery in around 30 minutes. Unless there is some technical problem with your battery or with your charger electronics, using a fast charger will not do any long-term major damage to your battery. However, if fast charging is not needed (e.g. overnight, when you sleep), I still recommend you to not enable it and use a lower power adapter (e.g. 5W power adapter)

Some extra tips:

• If possible, always use a Wi-Fi connection instead of a cellular network (Wi-Fi uses less power)
• When you use your iPhone, turn on "auto-brightness" (it adjusts your screen to lighting conditions automatically)
• Always update your iPhone to the latest software (updates often include evergy-saving technologies)
• Avoid charging or leaving your iPhone in hot environments (e.g. directly under the sun), because heat is the enemy number one for batteries. And if you notice that your iPhone gets hot when you charge it, take it out of the case (certain styles of cases may generate excess heat)
• If you are going to store your iPhone for a long period, keep it "half-charged" (50%)

* 20W or higher for iPhone 12

Charge it when not in use. Day and night. You can't overcharge it.