Bristle brushes in Illustrator can be found via the brushes panel and you can see all the current presets, though you can also go to the libraries section and select a library of them.
Select a preset you want to use, duplicate it via right side menu in the panel, double click the entry, edit the size, length, density, thickness etc (see preview to see the result of changes) and you now have your preset. I always find starting from an existing preset so much easier than just jumping in to creating a new one.
Or you can always create a fresh design by going to the new command in the panel and then selecting the resource you want.
There is a useful but fairly basic preview. Select a shape for your new, there are a number of shape options such as flat point, round blunt and many more. Set the size, length, density (though the action seems to be reverse to my mind), paint opacity etc. Tweak these and use the preview as a guide but the only real test is when you come to use the stroke on your art board. I generally always keep the opacity low. If you are not happy with your preset, you can always double click the entry and re-edit it.
You can use them once but the presets probably work best if you apply them over and over on top of the previously applied design and then vary the settings as well as the opacity and colors etc. A workaround to avoid creating multiple paths is to use the appearance panel - by using the appearance panel, a single stroke can be used and combined with others and colors as well as a mix of different opacities and blending modes as well as adding effects such as blurs etc. The appearance panel is a central part of the application that can be used to manipulate this type of resource in infinite ways, as well as all the others such as type, fills, symbols etc
You can select the design via the object menu and expand appearance. Also use the object menu and ungroup and you will end up with lots of individual paths that can be further manipulated, the colors changed, opacity modified and more. You will then see the underlying structure of the resource such as lines spread or long lines as well as the opacity etc and at that point, you can change the settings for the individual components at this point. The filled paths can also then have effects etc added as well as the results exported for use in Photoshop.
You can use them with the paint brush tool but it can also be used as a stroke for type as well as standard paths or perhaps the results of an expanded trace (personally I don't think the results are amazing but with a little trial and error, all kinds of overlays can be added to the image though to really work well you will have to set the image's fill to zero. To use the presets as a great source of designs with type, you must either expand the artwork or use the appearance panel and add to strokes. You can add multiple presets to the paths as well as type / text in that way, always just go to the right side menu and add new stroke and set the color and apply the offset and other effects
You can create some really beautiful multi-colored designs by using the presets with gradient swatches such as the gradient swatches and the gradients can be applied linear as well as radial etc Use the gradient panel to change the colors etc. Again, the best effect is if you add multiple strokes to layer the design. You can also use the artwork in layers and add the resource to one layer and then add a different presets with gradients or solid colors to another layer and so on. You can use the direct selection to select the individual points to twist and distort the artwork You can add effects to this set of designs as well such as adding zigzag which creates a subtle color variation. Or use the artworks with different blending modes such as lighten or darken or difference to create some truly amazing creations. https://youtu.be/WUx60MmQ1AI
The quickest way I have always found is just to create a line using the line segment tool and apply a preset to that and then double click the preset in the panel and display the settings and then just keep changing them and updating the currently selected path.
This is not a perfect solution and perhaps a couple of curved lines will be handy as well especially as sometimes you will get odd jags on a rapid turn of the artwork and that may not be visible when you are just trying out the settings. Changing the shape makes some difference and again that depends on the other settings so a round curve looks very little different to a flat curve on a straight line.
The preview and the selected line can be seen to change on release of the mouse and so you can with the length (seems to do very little beyond increasing the size and spread but to see the real results you would have to expand the artwork and see the paths.
Paint opacity in the panel makes the artwork a lot darker or transparent, so set low if you want to apply it over and over You can always blend into the mix the opacity and blending modes etc for the line.
Well, clearly the key reason for the existence of the presets is to draw with them so how you use the tools depends on what you want to achieve such as creating a smudged background or perhaps some strong lines around a path or just a loose sketch of a face, all are definitely possible with this tool set
You can combine multiple presets by using the powerful blend tool, simply create two strokes and then click on the first with the blend tool and then the next and you will have some amazing combinations of colorful lines. You can change the options by double clicking the icon or pressing return and you can then change the options to steps or distance. You can also tweak the individual strokes, as well, of course, changing the spine of the blend to create even more amazing artwork. https://youtu.be/WU_cD_KEXCU
You can select the generated stroke and with the direct selection tool you can move individual points which change the resulting design as well as remove and also add points. You can also apply live effects to the design such as a zigzag to create fascinating jagged strokes, you can also use other live effects such as roughen as well as transformation effects (distort and transform) to create all kinds of stunning repeated, but still, live effects.
roughen / transform effect https://youtu.be/LQs3bsbSBDE
\You can also add additional strokes via the appearance panel. Creating a very water color like softness to the artwork, especially if you combine multiple different color designs with live effects added.
You can use the presets inside a repeat object to create amazing radial designs and more. It is live and you can modify the instances, radius etc. You can also double click the artwork and then change the curve using the curvature etc inside the repeat object. https://youtu.be/dLGjMUKfVQo