This is lengthy, but right on. My second MacBook Pro’s logic board died after Catalina was released. I hated Catalina’s overzealous security, and Big Sur seems like an oversimplified yet buggy mess. I have unfortunately reached a point where, even with the release of Apple silicon Macs, I feel better off building a Windows PC. The OS was the reason for me to buy my first Mac; it’s now a reason to not buy my third.
Reduce Size of Mac Partition
Open Disk Utility, and select the top level of the internal SSD/HDD. Then click Partition at the top. Select your Macintosh HD partition, then under Partition Information > Size, reduce the size of the partition. Note that you are limited by how much space is already used (this amount is listed just below the Size text field). Click Apply.
Increase Size of Boot Camp Partition
If resizing Macintosh HD is successful, your next step is to boot into Windows. Open the Start menu, then type Disk Management. Look for your C: drive. Disk Management will only allow you to extend the C: partition if the free space you just created is to the right of it. If it is listed to the left, you must use a free 3rd party utility like MiniTool Partition Wizard Free. Here’s their tutorial on the Move/Resize Partition feature.
Please let me know if this short guide helps you.