iMac (21.5-inch, Late 2015) iMac16,2 crashes multiple times per day

You can save you and us a lot of time and effort if you do the following:

Please navigate to the Mac App Store or navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

There are many possible causes.

Can you add to the discussion what apps you were using when the crashes occur?

Do you have any anti-virus or so-called "cleaning" apps installed?

Please confirm your macOS version as 10.15

I would replace it with one these:

https://eshop.macsales.com/item/OWC/MS8U3SSDT1.0/

https://eshop.macsales.com/item/OWC/MOTGPWR/

I would not spend money opening it, these machines are very difficult to work on an external SSD will work very well and improve performance substantially.

You can use Carbon Copy Cloner to clone the internal HD to the new SSD. I would not recommend one of the BBY solutions, the solution I offered will work and the SSD is pre-formatted to work with your iMac directly out of the box.

You can use the evaluation copy to clone and if you don't already have a backup solution using Carbon Copy Cloner as a backup solution works great and if you already use Time Machine the it's wise to have redundant backups available too!

+1 on Carbon Copy Cloner. I use it because (a) it clones, not copies, the original drive to be bootable, and (b) it prompts you to create the important Recovery partition on the new drive. Most other methods do not mention the recovery partition and you end up not being able to run diagnostics and restore the OS easily.