How much battery run time you will have depends on how you are using the laptop. In my experience you will never get the hours listed by Apple in their advertising which came from pre-production hardware tests in a controlled environment on a system with no third party apps installed. Even Apple's own documentation is confusing:
MacBook Pro 14- and 16-inch - Technical Specifications - Apple
Up to 17 hours Apple TV app movie playback
Up to 11 hours wireless web
If wireless web browser gets you only 11 hours, then I'm not sure the 17 hours of wireless HD movie playback is possible although I do realize the CPU has special hardware instructions to help make the process more efficient for videos.
Most likely you have some sort of issue with third party software. The usual culprits are anti-virus apps, cleaning apps, and third party security apps since they interfere with the normal operation of macOS and are always running, scanning & monitoring the system. Also, Chromium based web browsers such as Google Chrome are notorious as resource hogs which also will cause a battery to drain quickly. The actual apps you are actively using while on battery will also play a huge part in how long you can run the laptop on the battery. Again, it is highly unlikely you will get 17 hours battery life with any activity unless you never installed any third party apps (even then I wouldn't bet on it).
You can try looking for possible software issues by running EtreCheck and posting the report here using the "Additional Text" icon which looks like a piece of paper. If you give EtreCheck "Full Disk Access", then the report will include more details which may provide more clues.
It never hurts to run the Apple Diagnostics to see if any hardware issues are detected.
Edit: Also make sure your battery is indeed at 100% when you do your tests because Apple's Optimized Battery Charging will many times leave the battery at 80% charge even after being connected to the charger for most of the day. Don't assume it is at 100% charge just because the charger has been connected long enough. It is easy to overlook this new behavior with macOS which is the default unless the user changes it.