First, rule out software. Open Activity Monitor (Cmd+Space, type 'Activity Monitor'), click the CPU tab, sort by %CPU. If something non-obvious is eating 90% of a core — a runaway Safari tab, a stuck Dropbox sync, an old Electron app — quit it and see if the fans calm down. Often they will. If CPU is low and fans are still screaming, your cooling system is the problem.
MacBooks pull air in through vents on the bottom and hinge, push it through a heatsink, and exhaust through the back. Over a year or two, those intake vents clog with pet hair, dust, and whatever was in your backpack. The fans spin up to compensate, but the heat can't escape. Everything thermally throttles. Performance tanks.
The least-invasive fix: flip the laptop over, find the intake vents (small slots near the hinge and on the bottom), and use a can of compressed air to blow dust out. Hold the can upright and use short bursts — long bursts cause the can to discharge liquid propellant, which is bad. Better yet, use an electric air duster (Meta, XPOWER, and OPOLAR make good ones for around $40–$70). They don't run out and they blow harder.
Important: hold the fans still while you blow air. If you let them free-spin at high speed they can generate a small voltage back into the motherboard. Stick a toothpick gently into the fan blade through the vent to lock it while you blast air through. This is the single detail that tells the YouTube videos apart from the ones that know what they're talking about.
If you're comfortable, remove the bottom case for a proper cleaning. You need a pentalobe P5 screwdriver (about $8 on Amazon) — MacBook bottoms use non-standard screws specifically to discourage opening them. Once it's off, you can see the heatsink, fans, and battery. Use compressed air on the heatsink fins from both sides. Don't touch the battery. Don't remove the fan screws. Just clean.
If the paste between the CPU and the heatsink has dried out — symptoms include sudden thermal throttling even in a clean system that's more than 4 years old — that is a repaste job, which is a 90-minute procedure and requires isopropyl alcohol, a thermal paste like Noctua NT-H1, and a confident hand. For most people that's the line where I'd hand it to a shop. iFixit has good guides if you want to try.
Worth mentioning: if you're on an Intel MacBook made 2016-2020, fans are going to run hotter than Apple Silicon, full stop. That's not a defect; that's the architecture. Cleaning helps, but it won't magically make a 2019 MBP run as cool as an M2.