Many, like myself, are constantly refreshing their grey matter, after long and distinguished (well long to be sure) careers. The only positive outcome of the COVID-19 pandemic I can think of is a deeper dive (sorry for the overused phrase) into distance learning. One of the sites I have found useful is www.nesoacademy.org. I discovered this site on YouTube years ago and downloaded the courses (if you are a Mac user, check out the 4k downloader app.) for archival. Neso Academy seems to have changed its "business" model in that they now offer "fuel" credits for paid content access, despite the material existing on YouTube. On the Neso Academy website, you can find material on Electronics, Signals and Sytems, Network Theory, Control Systems, and more. In my (humble) opinion, Neso Academy is a step up from Udemy.com (although Udemy is also a great site) in that the presentation(s) are more mathematical and theoretical than Udemy presentations. So, I find the re-learning a bit helpful as my career spanned IC design->Product Engineering->Yield Engineering->Test Engineering->software development->Embedded Systems. In short, I am a quasi-jack-of-all-trades master of some in the world of ID/Embedded System engineering with continuous DSP learning in my post-career. So, at the risk of exposing my ignorance, I thought I'd pass along another site that may help others in a similar situation :)
Thanks Dave, looks interesting.