Both user interfaces of course allow to start a user program.

On the keyboard the GO key is used to start the program (line 138, via a JMP to GOEXEC)

0678   1CA4 C9 13               CMP   #$13       ; RUN
0679   1CA6 F0 31               BEQ   GOV

0711   1CD9 4C C8 1D    GOV     JMP   GOEXEC  

The TTY CLI the G command handler at line 292 calls GOEXEC.

0864   1DEF C9 47               CMP   #'G'       ; GO EXEC
0865   1DF1 F0 D5               BEQ   GOEXEC

GOEXEC itself

0072   00EF             PCL     .BLOCK  1          ; PROGRAM CNT LOW
0073   00F0             PCH     .BLOCK  1          ; PROGRAM CNT HI
0074   00F1             PREG    .BLOCK  1          ; CURRENT STATUS REG
0075   00F2             SPUSER  .BLOCK  1          ; CURRENT STACK POINTER
0076   00F3             ACC     .BLOCK  1          ; ACCUMULATOR
0077   00F4             YREG    .BLOCK  1          ; Y INDEX
0078   00F5             XREG    .BLOCK  1          ; X INDEX

0841   1DC8 A6 F2       GOEXEC  LDX   SPUSER    
0842   1DCA 9A                  TXS   
0843   1DCB A5 FB               LDA   POINTH       ; PROGRAM RUNS FROM
0844   1DCD 48                  PHA                ; OPEN CELL ADDRESS
0845   1DCE A5 FA               LDA   POINTL
0846   1DD0 48                  PHA   
0847   1DD1 A5 F1               LDA   PREG
0848   1DD3 48                  PHA   
0849   1DD4 A6 F5               LDX   XREG         ; RESTORE REGS
0850   1DD6 A4 F4               LDY   YREG
0851   1DD8 A5 F3               LDA   ACC
0852   1DDA 40                  RTI    

What is happening here?

Note that GOEXEC is essentially build for SST operation! That is why all those registers are loaded from zeropage, where they were stored by the SAVE interrupt handler.

Program counter, Stack pointer SP, Process register PS, X, Y, ACC are initialized from the zeropage locations and the RTI transfers the CPU execution.
The RTI (return from Interrupt) restores all registers from the stack, including the Program counter.
So the user program starts at the current selected address (in POINTL, POINTH).

While this way of starting a program has the advantage that the program may start with user supplied values for the CPU registers, there is no guarantee that these zeropage locations contain meaningfull values. In fact, after a RESET the contents of RAM and therefore these start values are random. That can create a malfunction program if it does not itself initialize registers.
And not all do, like Microsoft Basic: the decimal flag is not cleared via CLD. And the program crashes if by chance the CPU is in decimal mode.

So before using the G command it is good practice to clear location 00F1, the Processor Status register to clear the decimal flag and 00F2, the stack pointer to FF.
A well behaving program should initialize stack and do a CLD just to be sure.

Returning to the KIM monitor can be done with a JMP START (1C4F).

RESET is executed when the user presses the RESET button, and the 6502 RESET input is triggered.

1210   1FF7             ;       ** INTERRUPT VECTORS **
1211   1FFA             
1212   1FFA              
1213   1FFA 1C 1C       NMIENT  .WORD NMIT
1214   1FFC 22 1C       RSTENT  .WORD RST
1215   1FFE 1F 1C       IRQENT  .WORD IRQT

0606   1C22 A2 FF       RST     LDX   #$FF       ; KIM ENTRY VIA RST 
0607   1C24 9A                  TXS   
0608   1C25 86 F2               STX   SPUSER
0609   1C27 20 88 1E            JSR   INITS
0610   1C2A             ;

  .. continue with TTY/KB selection

Subroutine INITS 

0963   1E88             ;
0964   1E88             ;          INITIALIZATION FOR SIGMA   
0965   1E88             ;
0966   1E88 A2 01       INITS   LDX   #$01       ; SET KB MODE TO ADDR
0967   1E8A 86 FF               STX   MODE        
0968   1E8C             ;
0969   1E8C A2 00       INIT1   LDX   #$00        
0970   1E8E 8E 41 17            STX   PADD       ; FOR SIGMA USE SAD
0971   1E91 A2 3F               LDX   #$3F        
0972   1E93 8E 43 17            STX   PBDD       ; FOR SIGMA USE SBDD
0973   1E96 A2 07               LDX   #$07       ; ENABLE DATA IN    
0974   1E98 8E 42 17            STX   SBD        ; OUTPUT
0975   1E9B D8                  CLD
0976   1E9C 78                  SEI
0977   1E9D 60                  RTS

What is happening here?

• After a hardware RESET (manually, the KIM-1 does not have an automated RESET circuit) the vector in the KIM-1 ROM points to $1C22 (1214)
• Stack initialized (606-610)
• INITS called to set hardware ports
• MODE of LED/display set to Address (Same as pressing AD on keyboard)
• hardware ports Port A set to input Datadirection register 0 (969 – 970)
• Ports PIA B PB0-PB5 set to output in Data Direction $3F = 0011111 (971-972)
• Port PB0 to PB3 set to 1, PB4 and PB5 to 0 by storing $07 = 00000111 to PIA B Data (973-974)
  This sets Row 3 output to 0, see the TTY/KB selection why