I had done vinegar and baking soda poured down them at the same time and let sit for 30 minutes and then flushed with hot water
Same as above.
As above, this does work, especially if one purchases Industrial Strength Vinegar used by painters for cleaning their precious brushes..I wipe my greasy pans out with newspaper or old rags..first before washing..this helps..also, use the vinegar and "boiling" water approach weekly..so that no build up accumulates..it is really a breeding ground for bacteria..ugh..
And never ever pour grease down a drain. Save soup cans, vegetable cans, coffee cans for this purpose.
Under sink there's usually a U, or J shaped pipe, you can unscrew the large nuts holding in place and manually scrape out the stuff. There's often sludge build up, and long hairs stuck in there. It's messy job. you'll wanna put a bucket or something underneath to catch water in the pipe.