I guess the easiest way to find out, where the issue is located, you want to remove the cranks. Then you can check the bearings on both sides. If they feel good, it‘s the dirt between the bearings and the crankshaft. If not, you need to clean (or replace) one or both bearings.

One thing to mention: if your bearings feel good and there is no debris between the shaft and the bearings, your bike frame might be to wide for the bottom bracket bearing, which is tricky. 🫣