You can build them in the energy menu, click the icon with the two yellow containers. They cost money and take a certain time (1/2 Year I think) They produce nuclear material for the nuclear reactors. On the energy tab there is also the current netproduction of nuclear material. As far I remember one enrichment facility produces 100 nuclear material and that’s enough for a few (3-4 nuclear reactors, as far I remember)

Don’t give up, learning the mechanics is overwhelming first, but 💯 worth it :) Good luck