Rostislav each of the protocol supported by flespi has its own specification. There are text protocols, binary protocols, some encoded protocols. Each protocol has its own implementation. Here are some details on how flespi protocol engineers implement it.
If you develop protocol by yourself and want it to be supported by flespi - use protocols with open specification: w.g. Wialon combine or Wialon IPS.
Or you can implement transport level protocol HTTP or MQTT and send data in JSON format.
E.g. this is Teltonika codec specification - https://wiki.teltonika-gps.com/view/Codec and this raw data sent by device
000000000000003608010000016B40D8EA30010000000000000000000000000000000105021503010101425E0F01F10000601A014E0000000000000000010000C7CF
will be parsed by flespi to
{"channel.id":1111,"din":1,"event.enum":1,"event.priority.enum":1,"external.powersource.voltage":24.079,"gsm.mnc":"24602","gsm.signal.level":60,"ibutton.code":"0000000000000000","ident":"356307042441013","position.altitude":0,"position.direction":0,"position.satellites":0,"position.speed":0,"protocol.id":68,"server.timestamp":1598611164.028156,"timestamp":1560161086}
