iMac 2011 as Mac mini 2018 display (Thunderbolt)?

Review the document: Use iMac as a display with target display mode - Apple Support

Target display mode is/was a feature available with late 2009-Mid 2014 iMacs. The display Mac has to be running macOS High Sierra or older and the signal producing Mac has to be running macOS Catalina (ca. 2019) or earlier.

This could only be done with a very few and very specific older iMacs and support was very limited. Read this topic for a 2021 perspective on this: Using iMac as an external screen - Apple Community

and this 2021 post by Allan Jones about software limitations:

TDM configuration - Apple Community

and read about Screen sharing Share the screen of another Mac - Apple Support

Related thread with information about Luna Display: iMac as display for M4 Mac mini - Apple Community

xiaojun129 wrote:

Can I connect iMac 2011 to Mac mini 2018 as a screen using thunderbolt cable and adapter?

Nope! That will not work, because the 2018 Mac mini does not support using the iMac as a display over a TB connection.

However you can use most aftermarket External Displays and/or an HDMI TV with the 2018 Mac mini.

Mac mini (2018) - Technical Specifications - Apple Support

Connect a display to Mac mini - Apple Support

xiaojun129 wrote:

Can I connect iMac 2011 to Mac mini 2018 as a screen using thunderbolt cable and adapter?

Not in any practical way.

If you wanted to make a Target Display Mode connection, you would need to

• Make a Thunderbolt connection, using $80+ of specialized Thunderbolt hardware ($50 Apple TB3-to-TB2 adapter, and a hard-to-find $30+ Thunderbolt 1/2 cable), and
• Downgrade the 2018 Mac mini to run Mojave or Catalina.

Since a 2018 Mac mini can run Sequoia, that would be a waste of the Mac mini. And for what?

A 2011 iMac screen would only have a resolution of 1920x1080 (21.5" iMac) or 2560x1440 (27" iMac). While the screen is a good-quality one, it's not like you would be getting a 21.5" 4K Retina screen or 27" 5K Retina screen.

...using thunderbolt cable and adapter?

Those alone could represent a significant portion of the purchase price of a decent external monitor. Plus you need a working external monitor already attached to install and set up most of the other external monitor "workarounds."