Technically, the dust limit is not fixed: it varies according to the type of output. 546 satoshis is simply the best known, for a p2pkh output. Being the type of output with the longest life, I suspect that some portfolios / blog articles / literature might treat it as a hard-coded dust limit.
To arrive at 546 satoshis, one must first know what "dust" means. A dust outlet is an outlet whose cost of expenditure is greater than its value. Even worse, an X BTC output that costs> X to spend is a dusty output. This is directly proportional to the amount of data required to spend an output, since the bitcoin fees are usually referred to as "bytes". The more you have to add bytes to your show to spend an output, the higher its dust threshold.
A very basic tx with 1 input p2pkh (~ 148 bytes) and 1 output p2pkh (~ 34 bytes) equals 182 bytes. The dust limit is 3 times this number (assuming a 1 satoshi relay fee), or
182 * 3 = 546 Sats.
For more complex txs, such as p2sh, this number is larger. For less space-hungry solutions, such as new segment options, this number would be lower.