My last betta did not seem to notice anything in his tank, it took him ages to find his food. I have cherry shrimps in my main tank so I added a female and two males to the betta's tank to see what he would do. He didn't notice them. The shrimps bred and I had to rescue baby shrimps from the old water at every water change - they went into the main tank as I didn't want the shrimps to take over the small tank.
My current betta does notice the shrimps, he chased them at first but couldn't catch them. I think there are now 9 adult shrimps in the tank with him, it's hard to count them. I see berried females regularly but since I've had this betta I have never sucked up a baby shrimp. I think this betta can catch the baby shrimps and eat them, but he no longer bothers chasing the adult shrimp.
However, several years ago I managed to get a woodlouse into the betta's tank, and the betta I had then became frantic. He chased it all over the tank attacking it when he could. I had to catch the woodlouse and remove it. A shrimp would have stood no chance with that betta.
The point of all this is - it depends on the betta.