Can I use my iMac as a monitor for my PC laptop?

The very restrictive software and hardware requirements for Target Display Mode (TDM) are laid out in this Apple article:

Use your iMac as a display with target display mode - Apple Support

Apple never intended, supported, not documented TDM for other then Mac-to-Mac, contrary to the tons of post-processed horse feed posted about TDM on the internet by misinformed writers lacking basic research skills or editorial support.

It is true that end users—not Apple,—accidentally discovered that SOME older non-Apple devices could run in TDM using 2009-2010 27" iMacs, but never when using any newer iMacs.

Apple started the deprecation of TDM ten years ago and threw the last shovel of dirt in in its face in early 2020. The serious power required and the limited display options compared to even a cheap free-standing monitor means it was never much more than an emergency workaround IMHO.

No

Some of the 2009 and 2010 iMacs supported Target Display Mode using DisplayPort input. Apple listed this as a feature in the Technical Specifications for the 27" ones.

Once iMacs got Thunderbolt in 2011, Target Display Mode required Thunderbolt input. At first, there were few Macs and no PCs that could provide that. Now, although there are many Macs and PCs that have Thunderbolt 3 / 4 ports, it would take $80+ in specialized hardware to make a connection to a Thunderbolt 1 / 2 port on an iMac that was old enough to support TDM. Then you'd find out that just having a Thunderbolt connection wasn't enough and that you had wasted your money.

Monitor prices have fallen over the years. At the time that the 27" Late 2009 iMac came out, Dell's standalone 27" 2560x1440 monitor listed for $1,200, and went for $1,000 even when on sale. These days, a basic but decent 27" 2.5K or 4K monitor might set you back $300 – $350.