error logs

It must be around a 2010 iMac if it is running High Sierra..

see > Identify your iMac model - Apple Support

Go to Applications > Utilities > Console > time of the error.

see > View log messages in Console on Mac - Apple Support

Unfortunately the macOS system logs are no longer useful for troubleshooting issues unless you are getting Kernel Panics. You will just go insane looking through macOS system logs these days...do yourself a favor and don't even look at them except for Kernel Panic logs. The regular log files are littered with many repeating entries many of which are cryptic, often scary sounding entries, with many entries being "sanitized" so they don't even contain the error details you want to review. I no longer look at the logs (except for the Kernel Panic logs) because it is a waste of time these days.

Even the Console app won't show all the logs these days. When I navigate to the relevant log folders, I find many more log files which should be displayed in the Console app side panel (they used to be displayed, but not anymore).

If the crashes were due to Kernel Panics, then you can find the Kernel Panic log files (if they saved successfully) in "/Library/ Logs/DiagnosticReports". The Kernel Panic files will have file names beginning with "kernel" and ending in ".panic".

It would not surprise me if you may have a failing hard drive, or some third party software causing a problem. If you have the 27" model, then a bad GPU could possibly cause a problem.

You can try running the Apple Diagnostics to see if any hardware issues are detected. Unfortunately the diagnostics won't detect most failures. You can check the health of the hard drive by using SMARTReporter. Feel free to post the health information here using the "Additional Text" icon which looks like a piece of paper. You can copy the information from the SMARTReporter app by clicking on the "Disk Checks" icon at the top of the SMARTReporter window, then clicking the "S.M.A.R.T." tab just below the "Disk Checks" icon, then click the "Additional Tools" tab midway down on the app. Click the "Attributes" button for the drive you want to check. Copy all of the health information in the window that appears and paste it here in the "Additional Text" field.

To look for possible software issues you can run EtreCheck and post the report here so we can review if for any obvious issues. The report may also include some log summaries which may provide some clues as it will tell you which apps have crashed or may be having issues.