I have a 2023 MacBook Pro and would like to use an old 2011 iMac as an external monitor. I have a thunderbolt to HDMI converter, since the new macs do not have a thunderbolt display port. I cannot get target display to work. Any tricks or advice?
The 2023 MacBook Pros have Thunderbolt 4 ports in the shape of USB-C ports which support Thunderbolt and Display Port protocols with the appropriate adapter.
However, the 2023 MacBook Pros don't support Target display mode as explained in the support document you posted from.
Use your iMac as a display with target display mode - Apple Support
The other Mac that you're connecting it to must have been introduced in
2019 or earlier and have macOS Catalina or earlier installed.
The 2011 iMac also has a very limited resolution compared to the 2023 MAcBook Pro, so the image would never look right even if it was possible.
Get an actual monitor that supports the newer resolutions for best results.
The 2023 MacBook Pros have Thunderbolt 4 ports in the shape of USB-C ports which support Thunderbolt and Display Port protocols with the appropriate adapter.
However, the 2023 MacBook Pros don't support Target display mode as explained in the support document you posted from.
Use your iMac as a display with target display mode - Apple Support
The other Mac that you're connecting it to must have been introduced in
2019 or earlier and have macOS Catalina or earlier installed.
The 2011 iMac also has a very limited resolution compared to the 2023 MAcBook Pro, so the image would never look right even if it was possible.
Get an actual monitor that supports the newer resolutions for best results.
While the 2011 iMac should support Target Display mode, you would need a Mac that "must have been introduced in 2019 or earlier and have macOS Catalina or earlier installed" to project to the iMac display, so unfortunately you can't use your new MPB. See Use your iMac as a display with target display mode - Apple Support
Even if Apple had not added restrictions on "the other Mac", you can't convert HDMI to Thunderbolt.
Making a Thunderbolt data connection would require a $50 TB3-to-2 adapter and a $30 TB 1/2 cable. But there is no point in getting this equipment, since even with proper cabling, the new Mac won't be able to use the old one as a TDM display.
Target Display: New 2023 Macbook Pro and Old 2011 iMac
