If you're curious which phones support L5, check out the table linked on this page (SCROLL DOWN to "GPSTest Database" for the up-to-date table, since the first one is out of date): https://barbeau.medium.com/9be4bbb83a7b
Is it really so few? Snapdragon got support for dual frequency in Snapdragon 855 released in 2018. Use the GPSTest app to see if your phone supports it.
If you want to have this feature you have to start looking at the high end models which are of course expensive.
Last time I've checked only Chinese brands like Huawei, Xiaomi or Realme seem to offer it at a more affordable price range.
According to this list[0] some Samsung Galaxy S20 does support it, as does the Pixel 5 (and 4 and 6). Google also lists both models as having L5 support[1].
Of course the best way of knowing if your phone supports it is to test it yourself.
It has a shorter wavelength so the correlation peaks are more narrow and hence more easily distinguishable. See this illustration [0] taken from this article [1]
To oversimplify: It's on a different frequency and so will be subject to different forms of error. You can average the results of L1 and L5 to get a more accurate position.
That's not how it works at all. L5 at 1176 MHz has a different ionospheric delay than L1 at 1575 MHz. Since the timing data is identical, you can calculate and remove the ionospheric delay if you can receive both signals at once.
Correlation peak width is an SNR property (and to a lesser extent dependent on receiver LO phase noise), not something that really corresponds to the RF band in use. Whichever signal is stronger (taking antenna gain into account) is the one that will yield better-quality baseband data.
L5 has a 10 times higher modulation ("chipping") rate than L1/2 which does lead to a narrower correlation peak width. However, it doesn't solve the multipath problem near tall buildings.
