Can I use my iMac 2019 as a monitor for my macbook pro

There's no way you're going to make that iMac work as a Target Display Mode display. Apple removed that feature when the first 27" 5K Retina iMac came out in Late 2014.

If your laptop is new enough, and your iMac is running Monterey or later, you could configure the 2019 iMac to be an AirPlay Receiver. I don't believe this will let you take advantage of the full 5K resolution of the iMac screen, and there may be issues found with software-based display connections, such as lag. But if you already own qualifying Macs, there would be $0 incremental cost to play with the feature.

System requirements for Continuity features on Apple devices - Apple Support

Set up your Mac to be an AirPlay Receiver - Apple Support

Not really, you may be able to use third party software to do that however you likely will not be happy with the performance. If your MBP needs an external display, please just buy one. You will find this less expensive (no expensive cabling or software), much more simple to setup and dead reliable. Please remember your iMac is a computer, it is not an external display for other devices.

Sorry, no.

There are some software based mirroring solutions, but the lag makes it all but unusable for anything but the simplest of tasks.

hrh226 wrote:

My iMac 2019 has been running extremely slow and crashes all the time.

If this is due to a hardware failure (e.g., failing hard drive), it could be a bit difficult (or at least costly) to repair.

But have you considered the possibility that the problem is something that could be fixed fairly cheaply, e.g.,

• By uninstalling "cleaner", "security", anti-virus, etc. applications that consume system resources?
• By backing up your system to an external drive (or two), erasing your internal drive, doing a "clean install" of macOS, and using Migration Assistant to pull your data back in from your backups?
• By installing more RAM, if the Memory Pressure graph in Activity Monitor shows a lot of yellow and red when you're running your regular workload?
• By using an external SSD as your startup drive, if you are currently running off a 1 TB Fusion Drive? The 1 TB Fusion Drives in the 2019 iMacs had a very skimpy 32 GB of SSD space.
• By considering using an external SSD as your startup drive, if you are currently running off a 2 or 3 TB Fusion Drive? (Those have 128 GB of SSD space, so are more likely to be performing acceptably.)