The questions are:

How we can change the extrusion velocity in the G-code? ( I put in bold the command we think are the ones who change the velocity of extrusion)

We are changing the rotational velocity of drivegears or the velocity at which the filament exits from the nozzle?

Thanks in advance.

M73 P0 R1
M73 Q0 S1
M201 X1000 Y1000 Z200 E5000 ; sets maximum accelerations, mm/sec^2
M203 X200 Y200 Z12 E1000; sets maximum feedrates, mm/sec
M204 P1250 R1250 T1250 ; sets acceleration (P, T) and retract acceleration (R), mm/sec^2
M205 X8.00 Y8.00 Z0.40 E1000; sets the jerk limits, mm/sec
M205 S0 T0 ; sets the minimum extruding and travel feed rate, mm/sec
M106 S0; set fan
M862.3 P "MK3S" ; printer model check
M862.1 P0.4 ; nozzle diameter check
M115 U3.9.1 ; tell printer latest fw version
G90 ; use absolute coordinates
M83 ; extruder relative mode
M104 S240 ; set extruder temp
M140 S0; set bed temp
M190 S0 ; wait for bed temp
M109 S240 ; wait for extruder temp
;G28 W home all without mesh bed level
;G80 ; mesh bed leveling
;G1 Y-3.0 F9000.0 ; go outside print area
;G92 E0.0
;G1 X60.0 E1.0 F6000.0 ; intro line
M73 Q0 S1
M73 P0 R1
;G1 X100.0 E1.0 F6000.0 ; intro line
;G92 E0.0
;M221 S95

; Don't change E values below. Excessive value can damage the printer.
M907 E430 ; set extruder motor current
G21 ; set units to millimeters
G90 ; use absolute coordinates
M83 ; use relative distances for extrusion
M900 K0.05 ; Filament gcode LA 1.5
M900 K30 ; Filament gcode LA 1.0
;BEFORE_LAYER_CHANGE
G92 E0.0
;0.2

G1 E-0.80000 F10800.00000 ; retract
G1 Z170.000 F10800.000 ; lift Z
;AFTER_LAYER_CHANGE
;0.2
G1 X170.077 Y150.000 ; move to first skirt point
G1 Z170.000 ; restore layer Z
G1 E0.80000 F10800.00000 ; unretract
M204 S1000 ; adjust acceleration

G1 F600.000
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;
G1 X170.077 Y150.000 E0.50000 ;

G1 F8640.000
G1 X170.077 Y150.000 E-0.76000 ; wipe and retract
G1 E-0.04000 F10800.00000 ; retract
G1 Z170.000 F10800.000 ; lift Z
G1 X170.077 Y150.000 ; move to first perimeter point
G1 Z170.000 ; restore layer Z
G1 E0.80000 F10800.00000 ; unretract
G1 F3000.000

; stop printing object 
G1 F8640.000;_WIPE
G1 X170.077 Y150.000 E-0.39162 ; wipe and retract
G1 F8640.000;_WIPE
G1 X170.077 Y150.000 E-0.36838 ; wipe and retract
G1 E-0.04000 F2100.00000 ; retract
G1 Z170.000 F10800.000 ; lift Z
; Filament-specific end gcode
G4 ; wait
M221 S100 ; reset flow
M900 K0 ; reset LA
M907 E538 ; reset extruder motor current
M104 S240 ; set temperature next attempt
M140 S0 ; set heatbed next attempt
M106 S0; set fan next attempt
G1 Z170.000 ; Move print head up
G1 X170.077 Y150.000 F3000 ; home X axis
M84 ; disable motors
M73 P100 R0
M73 Q100 S0