Weeds on the lawn: just use a lawnmower each week, the grass will usually handle being cut on a weekly basis much better than any weed.
Weeds between tiles / slabs or on gravel: just pour boiling water over them. The weeds will become mushy and die within 1-2 days. Repeat every 6 weeks during summer.
Source: we bought a house with a garden full of goutweed [0], which I consider the final boss of any garden owner, and which we have in control now through regular mowing / hot water. Goutweed will just laugh at any herbicide you throw at it, and regrow from its underground rhizomes. I also doesn't seem to require sun, because I have seen plants grow to a height of 10cm completely underground. The joke in my family is that it could grow on foreign planets. As Wikipedia dryly puts it: "Once established, goutweed is difficult to eradicate."