Seeds are produced where the flowers have been - marigolds are easy, they're large seeds, if you mean English marigold (Calendula). So if you want to collect seed from any plant, that's where it'll be. Some plants are sterile though, so don't produce seeds after flowering.
Cuttings vary from plant to plant - optimum times are different, and some root better if they're soft, some better if they're hard, or some work well with layering. Strawberries produce runners - these runners root into soil if they're in contact with it.
Iinformation on this subject can be obtained from most good gardening books. particularly the encyclopaedia type ones such as the Reader's Digest Encyclopaedia of Garden Plants and Flowers. You can also get specialist books on the subject of Propagation, which should show techniques - Google is useful too - if you know, for instance, you need a semi ripe cutting from a particular shrub, because that's what's recommended for it, typing semi ripe cutting into Google will produce various things in which will be included a description of the procedure.

My favourite book is Dorling Kindersley Plant Propagation. Packed with info on how to propagate everything!

I've just been searching for the title you mention, Volunteer....is it an older book as I can't find it listed anywhere....even on the RHS Dorling Kindersley site.

Which (the consumer association) have a book on propagation, or at least they used to...

I have a book called' the complete book of plant propagation' by graham clarke and alan toogood published by ward lock. Its a few years old but very comprehensive. The range of ' expert' books have sections in them as well. I'd go to a good bookshop or garden centre and see which stye suits you-- charity shops also carry a good range of gardening books too

Yes, Pam.....I'm always on the lookout for gardening books when we get to the charity shops...have picked up a couple, mainly about garden structure, design. should broaden my selection and look for propagation topics as well.

The author of the DK book is Alan Toogood, Whistonlass. It's an RHS book. RRP £16.99, but some discount shops (British Bookshops) sell it for about £9 in paperback. It would be on my Desert Island Booklist!

Thank you for the book details....I will keep out an eagle eye for it (likely the paperback edition to save a few pennies....more money for plants that way...lol)

Thanks, Volunteer.