A lot of modern RC toys use 2.4 GHz chips originally intended for other purposes, such as wireless mice and keyboards. Generally each manufacturer of one of the original designs chose their own modulation and packet framing details uniquely - it would be possible to make a universal chip that handled all of those, though short of going to a software defined radio implementation it is not clear that there is anything currently on the market which does. Three major chips that have been used are the Cypress CYRF6936 type, the A7105 type, and the NRF24L01 type.
However, low-cost consumer electronics is a world of copycats, so the NRF24L01 style has received a lot of imitation - both in devices sold under legitimate differentiating brand names, such as from Beken or on modules from Hope, and also in falsely labelled counterfeits that are still generally, if imperfectly functional. Additionally, Nordic's BTLE SoCs can typically operate in an NRF24-compatible mode. And the XN297 part that has seen heavy usage in the past year has compatible on-air modulation, but some differences in framing and data handling.
Even when using using the same chip, there are often a number of differences in the protocol which different products use to transmit data over it. This covers questions such as when to hop to a new frequency, out of what collection, and how often to transmit. It also includes questions such as how the data from a given control function is encoded in a packet, and even basics such as which way a control actuator or electronic throttle should respond to which direction of control stick movement. Many products also have a process of "binding" during which transmitter and receiver agree on a set of frequencies or address codes to use with each other. But using inexpensive SPI logic analyzers, many enthusiasts have been able to reverse engineer these details and publish open source projects implementing compatible encoders or decoders, often for a variety of protocols in a single program. Typically this takes the form of an ATmega or STM32 connected via SPI to a generic RF module containing the required 2.4 GHz chip, but in a few cases the output is instead a serial stream to a more sophisticated RF module that implements details of the proprietary air protocol. The individual chipsets are inexpensive enough that more than few hobbyists have modified their favorite control transmitters to simply contain modules for each of the three major families, with software then covering the finer details.
In addition, some of the finer degrees of differentiation between semi-compatible chips have been figured out to the point where software can achieve compatibility. For example, it is possible via data manipulation to have a genuine NRF24L01+ or close compatible transmit in a way that can be received by an XN297, though it is not yet clear if the reverse will be true.
Obviously, what is used in a fast-evolving price-driven consumer market changes rapidly. There already seems to be a trend away from having a distinct radio IC connected to an MCU via SPI, towards integrating the radio and MCU functionality in the same part.