is there a iMac with a 27 inch screen?

Not any more. Apple made 27" Macs from Late 2009 through 2020. The ones that Apple released between Late 2014 and 2020 had Retina 5K screens.

The closest current equivalent to a 27" iMac would be

• A Mac mini
• A 27" 5K Apple Studio Display
• An Apple Magic Keyboard and Apple Magic Mouse

The 27" 5K Studio Display is rather expensive by 27" 5K iMac standards, but there are now several other brands of 27" 5K (5120x2880) monitors out there.

Not currently. Apple stopped making them.

(259085)

There was one. I still have a 2010 model. I loved the big screen but could no longer get software or OS updates so I bought a new 24" model and like it.

Apple has not made a Mac with a 27" display for about 5 years. What a LOT of users that need to upgrade are doing is buying current (M4 based Mac mini) and then selecting the display of their choice. For example, you can now get a M4 Mac mini with 16GB of RAM and a 1 TB SSD for $999, add another $300 for a high quality 27" display (there are many many to choose from) and you have an extremely powerful Mac.

Another option is Apple sells refurbished Macs (sorry they stopped selling 27" iMacs years ago), a refurbished Mac is identical to new, has the same warranty as new, is eligible for AppleCare and can save hundreds of dollars. This means there is no downside. If you decide on a Mac mini (model year 2024 with the M4 CPU) you can get some very very nice machines.