Metric conversion for sizes (CM, INCHE, MM) in a dropdown - HELP?

[LucieFapoosy]

Hi All,

First time posting on here, but here goes. I'm looking to add a dropdown button or a feature that works im a similar way to this website.  https://www.ideelart.com/

As you can see from the picture, in the top right corner, in the same way the currency works, they have a metric in place so you can swap between your preference and this changes everything on the site to match.

My website sells transfers in specific sizes. Here are 2 pages for reference
- https://www.universaltransfersuk.com/products/custom-dtf-transfer-with-gang-sheet-builder
- https://www.universaltransfersuk.com/products/struggle-bus-get-in-bestie-8-dtf-transfer-132

I'm not sure if it would be as simple, as editing how the currency thing works, and adding in all my set sizes as I took a look at the source code like the detective I am haha and on line 588 I saw the word inch and was able to see coding for the dropdown. They aren't using Shopify so it is probably custom code etc.

Another feature this changed, is under the title, much like in Shopify you can show the price, owner etc, this was showing a size metric and it was linked to the dropdown. If changed, the metric there changed.
Currently I have sizes in alot of my products titles, I don't really want to add  "Princess 5"/Xcm/Xmm/Xwhatever so having a custom thing like this is much more inclusive and on brand for me and helping my customers as much as possible.

Open to all ideas or any help/suggestions or if anyone knows of someone who does custom app work please message below!