It is important to know that the tabs/ino files are concatenate into one by the builder. The order of the concatenation is the same as the order of tabs in IDE. First is the main ino file with the same name as the project. The rest of the files is concatenated in alphabetical order.
The concatenated ino file determines the visibility of variables. All ino files 'see' the variables in the main ino file, but variables in other ino files are visible only in ino file with names later in alphabetical order.
For functions the builder generates function prototypes to the beginning of the resulting cpp file, so as usual all functions are visible everywhere in all ino files.
One way to work with additional tabs is group is to write xySetup() and xyLoop() functions in the xy.ino file and then call this functions from the main setup() and loop(). My main Arduino project is created this way, so it is a big and complex example of this approach.
To illustrate, this is the resulting cpp file entering the cpp part of the build toolchain (scroll it to end):
#include "Arduino.h"
#define I2C_ADC121 0x50
#include <Wire.h>
extern const unsigned long EVENTS_SAVE_INTERVAL_SEC;
extern const char eventLabels[];
extern const char* eventLongLabels[];
extern const char* eventLongLabels[];
#define EVENTS_FILENAME "EVENTS.DAT"
#include <Grove_LED_Bar.h>
#include <StreamLib.h>
#include <TimeLib.h>
#include <MemoryFree.h>
#include <UIPEthernet.h>
#define ETHERNET
#include <ArduinoOTA.h>
#include <SD.h>
#define FS SD
#define Serial SerialUSB
#include "consts.h"
#define BLYNK_PRINT Serial
#define BLYNK_NO_BUILTIN // Blynk doesn't handle pins
#include <BlynkSimpleUIPEthernet.h>
#include <RTCZero.h>
#define STATS_FILENAME "STATS.DAT"
void balboaSetup() ;
void balboaReset() ;
void balboaLoop() ;
void battSettLoop() ;
boolean battSettRead(FormattedPrint& out) ;
const char* bit2s(int value, byte mask) ;
int battSettControl(boolean chargeControlOn, boolean dischargeControlOn) ;
int battSettSetLimit(byte reg, int limit) ;
void beeperLoop() ;
void alarmSound() ;
void beep() ;
void beeperTone(int freq, uint32_t time) ;
void blynkSetup() ;
void blynkLoop() ;
void updateWidgets() ;
void buttonSetup() ;
void buttonLoop() ;
void csvLogSetup() ;
void csvLogLoop() ;
void csvLogPrintJson(FormattedPrint& out) ;
void elsensSetup() ;
void elsensLoop() ;
boolean elsensCheckPump() ;
byte overheatedSecondsLeft() ;
int readElSens() ;
unsigned short elsensAnalogRead() ;
void eventsSetup() ;
void eventsLoop() ;
void eventsWrite(int newEvent, int value1, int value2) ;
boolean eventsSaved() ;
void eventsSave() ;
byte eventsRealCount() ;
void eventsPrint(FormattedPrint& stream) ;
void eventsPrint(FormattedPrint& s, int ix) ;
void eventsPrintJson(FormattedPrint& stream) ;
void eventsPrintJson(FormattedPrint& stream, int ix) ;
void eventsBlynk() ;
int eventsCompare(const void * elem1, const void * elem2) ;
void ledBarSetup() ;
void ledBarLoop() ;
void manualRunLoop() ;
byte manualRunMinutesLeft() ;
void modbusSetup() ;
boolean modbusLoop() ;
void modbusClearData() ;
boolean requestSymoRTC() ;
boolean requestInverter() ;
boolean requestMeter() ;
boolean requestBattery() ;
boolean modbusError(int err) ;
int modbusRequest(byte uid, unsigned int addr, byte len, short *regs) ;
int modbusWriteSingle(unsigned int address, int val) ;
int modbusConnection() ;
void pilotLoop() ;
unsigned short power2pwm(int power) ;
void setup() ;
void loop() ;
void shutdown() ;
void handleSuspendAndOff() ;
void clearData() ;
boolean handleAlarm() ;
boolean restHours() ;
boolean turnMainRelayOn() ;
boolean networkConnected() ;
void statsSetup() ;
void statsLoop() ;
int statsEvalCurrentPower() ;
void statsAddMilliwats() ;
void statsSave() ;
int statsConsumedPowerToday() ;
void statsPrint(FormattedPrint& out) ;
void statsPrint(FormattedPrint& out, const char *label, Stats &stats) ;
void statsPrintJson(FormattedPrint& out) ;
void statusLedSetup() ;
void statusLedLopp() ;
void statusLedShortBlink() ;
void susCalibLoop() ;
void telnetSetup() ;
void telnetLoop(boolean log) ;
void valvesBackSetup() ;
void valvesBackReset() ;
void valvesBackLoop() ;
void valvesBackStart(int v) ;
boolean valvesBackExecuted() ;
unsigned short valvesBackTempSensRead() ;
void watchdogSetup() ;
void watchdogLoop() ;
void WDT_Handler(void) ;
void webServerSetup() ;
void webServerLoop() ;
void webServerRestRequest(char cmd, ChunkedPrint& chunked) ;
void webServerServeFile(const char *fn, BufferedPrint& bp) ;
void printValuesJson(FormattedPrint& client) ;
void printAlarmJson(FormattedPrint& client) ;
const char* getContentType(const char* ext);
void wemoLoop() ;
boolean wemoPowerUsage() ;
int wemoRequest(const char* service, const char* action, const char* param, const char* value, char* response, size_t size) ;
#include "Regulator.ino"
#include "Balboa.ino"
#include "BattSett.ino"
#include "Beeper.ino"
#include "Blynk.ino"
#include "Button.ino"
#include "CsvLog.ino"
#include "ElSens.ino"
#include "Events.ino"
#include "LedBar.ino"
#include "ManualRun.ino"
#include "Modbus.ino"
#include "PowerPilot.ino"
#include "Stats.ino"
#include "StatusLed.ino"
#include "SusCalib.ino"
#include "Telnet.ino"
#include "ValvesBack.ino"
#include "Watchdog.ino"
#include "WebServer.ino"
#include "WemoInsight.ino"