On Windows 11 you will live in Task Manager (what is eating the CPU (Central Processing Unit) or RAM (Random Access Memory)), Event Viewer (what failed and when), Services (what should be running but is not), Settings → Windows Update, Device Manager (drivers and unknown devices) and Disk Cleanup / Storage Sense. Add the Reliability Monitor: it gives a plain timeline of crashes that Event Viewer buries.
On macOS the equivalents are Activity Monitor, Console, System Settings → General → Software Update, System Information (hardware and extensions), and Disk Utility with First Aid. Safe Mode and a Non-Volatile Random-Access Memory (NVRAM) reset are still worth knowing on Intel Macs; Apple Silicon Macs use a different recovery path you hold the power button to reach.
User profiles cause more mystery faults than hardware. When one user has a problem on a machine and another does not, the profile is the suspect. When everyone on the same machine has it, the machine is the suspect. When everyone on every machine has it, look at the network or the cloud service.
Keep a small kit: a bootable USB installer for both platforms, a spare Universal Serial Bus (USB) hub, a known-good Ethernet cable, a USB-C to display adapter, and a compressed-air can. Half of 'the computer is broken' is a dock, cable or adapter.