Those are great questions. Welcome aboard. We have swaps that Kyiav and I host for charm squares (individual squares that make up a unique charm pack and fat quarters...sometimes for strips as well known here as noodles). For the swaps, they are prewashed but in the stores they are not. As mentioned stores generally sell a group of fabrics from the same line for these although lately I've seen remnents cut into charms. If you're using charms, there are plenty of free patterns online and you can check the board for suggestions. I keep some ideas on the charm swap thread each month. Strips can also be used to cut down your time in creating a quilt and if you're trading them, you can get a scrappy look without having to buy a lot of different kinds of material. The FQ swap is helpful for stash building in addition.
I have not seen the charm pks, jelly rolls, or layer cakes at discount stores but quilt shops only. There are occasional fat quarter packs there, however. I have seen some online stores offer a free pattern with purchase of a pack or roll but they do not automatically come with patterns.
I haven't been able to do the BOM but do use some of the tutorials for new blocks. Basically, you can use them for whatever you like. You can do a small or large project, a sampler quilt, practice blocks to learn new techniques, or trade/donate them.