This is going to surprise no one, but people can put fake things on the internet.
If you're on the internet and you see some tiktok atelier or instagram influencer showcasing a super easy technique, but you just can't make it work right, it might not be you.
Are they demonstrating their "never clip a princess seam!" technique or their "magical 1-handed rolled hem using nothing but a cigarette and a ball point pen" strategy on muslin fabric?
Muslin is often used for demonstration and test garments and it's not inherently dishonest. It's very useful, because it's stable, doesn't fray a lot, doesn't shift, but will sharply hold a crease when ironed and will stay put if you iron it into a curve or on the diagonal. If someone's hemming a circle on muslin, they can press the turned up area, and a good muslin will stretch at the long edge and compress at the short edge, making a very smooth and flat seam that will just stay there. Not all fabric stretch, compress, or hold a crease.
Using muslin in a demonstration isn't cheating at all. The sewing world uses it in a lot of places for a lot of reasons. The way that it works when it's stretched or compressed or ironed is part of why we do use it. But if you see someone showcasing a super amazing technique on a piece of muslin, and then you can't make it work, it's not you. It might be that they showed a technique that doesn't work on all fabrics, and your fabric isn't one of the ones it works on. People don't disclose this in short-form videos because there's not enough time and people don't go viral by giving the viewer all the information that they need.
Anyway, it's all a conspiracy, don't trust anyone, drink some water, don't hunch your back over your work, and don't blame yourself when something looked way easier on Social Media Application Tick Tock than it does in front of your sewing machine on your kitchen table.
