I'm guessing htis is a "standard" commode and not a power-assist flush?
With the top taken off the back of the tank, is it not apparent where the water is "shooting" from as it fills?
In some older toilets, the water enters the tank through a fill valve. The open/close function of this valve is controlled by the ball float. The float rises, the valve closes...vice-versa.
Again, in older tanks, the water comes out of that valve and flows through a flexible tube, about 1/4" diameter. That tube fills a standpipe, and is held to the top of that standpipe by a small clip.
Check that tube and the clip. Sometimes the tube can come loose from the clip, thus allowing the tube to shoot water where it's not supposed to.
If your toilet is newer, those fill assemblies, to me, are a mess. Too much plastic, too many inadequate joint assemblies and ultrasonic welds that, on occasion, fail.
If it's not obvious that something just needs tightening or tweaking, it's often easier to replace the whole fill assembly. They're not expensive and are readily available at home centers.
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