Questions about the somewhat-unique number stamped into your bike's frame, often under the bottom bracket, but sometimes in a chainstay or on a rear dropout. Occasionally the serial can be elsewhere on the frame. It is rare for forks to have a serial number though not impossible. Other components may have their own serial number, but this is mostly in high-end parts.
Serial Numbers are mostly-unique identifiers for tracking a bicycle frame through manufacturing and sale, and for identifying the frame for after-sales support or in recovery of stolen good.
Serial numbers are NOT a VIN (vehicle identification number)
A given serial number does NOT show the manufacturer.
Serial numbers are not unique between brands, and might even have duplicates inside a brand, depending on the quality of the documentation.
Your frame's serial number might encode information about when the frame was made, or which factory, but these details depend on you knowing the manufacturer, AND that manufacturer using an encoding system AS WELL AS that system being known and understood.
The older a bike is the less-likely the serial number can be resolved to something useful. Mongoose is a prime example of where there is no system, and many records were lost over time.
There are numerous web sites where owners have attempted to make sense of their serial numbers. Some are useful while, some contain wild speculation with no evidence. An example of one maker documenting his work is https://davemoultonregistry.com/ He lists known live bikes, not the details from his original sales records.
The main purpose of a serial number for the owner, is identifying your particular bike. In the event of recovery after a theft, the serial number proves that it is your bike, but only if you have a copy of that serial. Sales paperwork may show the S/N but if you bought the bike used then you won't have that. A photo of your bike and its serial number is a good idea.
Serial numbers can be used to register a bike frame with your insurance company, and with various web-based registers of bikes. Anything you can do to help ease recovery of your bike, is worth doing.
If your bike really doesn't have a serial number, then it might be a hand-built frame from a small framebuilder. Or it could have been ground off. Or perhaps its just under all that dirt. There's also a chance it was painted while dirty and simply is invisible. Lastly, your frame might have had a serial number on a sticker, along with a barcode. Cannondale has done this since the 2000's, and if the sticker is damaged then the serial number is possibly gone.