The chainrings are bolted on to the spider so that they can be replaced if worn or damaged or to provide different gearing.
Replacement chainrings must be chosen with a bolt-hole count and correct width that matches the spider, chain and rear sprocket.

Bolt circle diameter (BCD)

Most bicycles have removable chainrings (only real cheap bikes have the chainring welded to the crankarms), to allow for replacing the chainring when worn, or to change the gear ratio provided. The holes on the crankspider arms used for attaching a chainring can have different dimensions, we refer to this as the bolt circle diameter, commonly abbreviated as BCD (in Dutch 'steek)'. Cranks designed to mount one or two chainrings will almost always use a single bolt circle diameter. Cranks designed to mount three chainrings will almost always use two different bolt circle diameters; the larger to mount the two outer rings and the smaller to mount the inner ring. Most modern two-chainring cranks use either a 110mm or 130mm bolt circle diameter.

Power Transfer

Chainrings for fixed gear / singlespeed. The chainring is the large ring on a crank that a chain attaches to, in order to transfer energy to a wheel. It consists of one or more sprockets that are driven by the cranks and pedals of the bicycle
So the fixedgear chainring is a key component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel.

Fixedgear chainrings engage the singlespeed chain to transfer power to the wheel. Chainrings have teeth spaced to engage every link of the bicycle chain as it passes over.

Chainring widths singlespeed / fixedgear
Mostly 2 different widths are used for singlespeed
1/8" mostly used for track, bmx.
3/32" is a bit smaller and also used for singlespeed and fixed

Chainring material

Chainrings are mostly constructed in aluminum alloy, titanium or steel.

Known brand for singlespeed / fixedgear chainrings

Brands you will find for replacement chainrings for fixedgear will be BLB, BLB has the BLB track chainrings and BLB freeride chainrings. Miche is creating singlespeed and fixedgear chainrings in a 135BCD and 144 BCD. Ridea chainrings are beautiful fixie chainrings of high quality. Shimano has a nice selection of professional track chainrings. Surly is making stainless steel sturdy fixie chainrings. All-City has fixedgear chainrings in their collection as well.