Saw this on my screen how do i find out which third party app got the legacy system extension?

The system extension will become incompatible with future version of macOS. It will probably function fine with current versions. Your contact with the developer may show interest for updated software for future macOS versions (Big Sur) in due time.

Devguru Co., Ltd. seems to be the maker of software for connecting iPads to Macs for various uses. If you still use that, then you may be interested in trying to persuade the developer to continue supporting their tools for future macOS versions.

You saw what on your screen? You post does not include a reference to any legacy system extension, that we can see.

(For some extensions, you will have to google the extensions name to find the attached developer, and then contact them about any newer versions that are compatible with your current OS version.)