Which teacher has a good understanding of the working principle of gas detectors? Thank you!

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What is the working principle of alarms such as electrochemical detectors, photoionization detectors, and flame ionization detectors? What is its selectivity? I understand something about the principle of catalytic combustion gas detectors, so I can design them flexibly. However, the principle of toxic gas detectors is completely confusing when I look for information online; could any teacher explain it to me? It’s best to have your own understanding, rather than copying from the Internet. GBZT223-2009 specifies that toxic gas detection and alarm devices in workplaces \"should\" be selective; then what does Appendix A list, which contains a bunch of non-selective detectors, mean? The Design Code for Detection and Alarm of Flammable and Toxic Gases in the Petrochemical Industry (GB50493-2009) states that detectors should be installed for those substances listed as highly toxic hazardous materials. However, GBZT223-2009 does not mention any selective detectors for such gases. What should be done in such cases? For example, should detectors be installed for carbon disulfide? Does anyone among my friends have their own opinions on this? Thank you!