Is there an optimal setting for training? Should I train all firms, even factories with new technology being developed every 3 years? I never train R&D units as it seems to take forever and training costs a lot of money.

I also notice that I will experience a shortage of some input periodically ...

Over the years, there are many who posted their strategies, but spread out in different places. Let's do a headcount and preferably added links that can show pictures/saves as reference, or just share your strategies or tactics in replies. I'll started a list below with brief descriptions added ...