To make these brushes work for you, you need to reverse-engineer them. Marc’s pack (approximately 30+ brushes) is not about quantity; it is about efficiency. He has three pillars of brush design:

How to install Illustrator Brushes. - True Grit Texture Supply

Designed to create both lost and found edges effortlessly.

While Marc Brunet offers high-quality free resources, the "Advanced" brush set is generally a paid product, whereas the "Starter" pack is available for free.

So, how can artists access Marc Brunet's advanced brushes for free work? The process is straightforward:

Struggling with digital painting? I’ve released a free sample pack of my Advanced Brushes for Photoshop.

He dropped a link. "The 'Advanced Pack' is still $39. The 'Free Work' demo file—the layered PSD of this knight—is downloadable right now for zero dollars. Go study it. Reverse-engineer every stroke. Steal my process, not my assets."