If you install NOOBS, then the expected behaviour is for it display a splash screen at startup asking you to "press shift for recovery."
If you have only installed one additional OS (e.g. Raspbian), then after a short delay it should boot straight into Raspbian.
If you have installed multiple OSes, then it will enter the NOOBS boot menu instead to offer a list of OSes for you to choose which one to boot.
It may also go directly to the NOOBS recovery menu if, for example, you have a HAT attached which is altering the default state of some GPIO pins, you press the Shift key, or the mouse button, or added "forcetrigger" to recovery.cmdline. There may be other reasons detailed in the NOOBS documentation, or referenced from the linked post above.
The "autoboot.txt" file is a firmware feature and can be used to bypass NOOBS altogether.
It should be placed on the first partition of the disk, where NOOBS is installed. This is normally /dev/mmcblk0p1 with the label of "RECOVERY".
autoboot.txt just requires one line:
Where [partition] should be replaced with the partition number of the boot partition of the required OS.
In the case where you have installed Raspbian only, then this should be 6:
However, once you have set this file, Raspbian will always boot and NOOBS will no longer be available. If you ever need to go back to NOOBS, you must remove this file. This is easily done by moving your SD card to Windows and editing it there (Do not accept any windows advice to reformat any drives because they are not accessible). You can also do it from Raspbian, but it requires mounting /dev/mmcblk0p1 on a mount point.