In many scenarios you might see 2 or 3 different IDs on a call, but Analytics will use the first ID present from that list as the official visitor ID, and split that value across the post_visid_high and post_visid_low columns. For example, if you are setting a custom visitor ID (included in the "vid" query parameter), that ID will be used before other IDs that might be present on that same hit.
In terms of cookies it depends if you have implemented the Visitor ID services or not but the general guidance for cookie expiration can be found here:
This cookie is stored at the domain of the image request - typically 2O7.net if you are using third-party cookies, or your domain if using first-party cookies.
Note: Each Analytics visitor ID is associated with a visitor profile on Adobe servers. Visitor profiles are deleted after 1 year of inactivity regardless of any visitor ID cookie expiration.
Cookie Name: s_fid
Fallback unique visitor ID time/date stamp
This cookie is used to identify a unique visitor if the standard s_vi cookie is unavailable due to third-party cookie restrictions. Not used for implementations that use first-party cookies.
This cookie is stored on your domain as a first-party cookie.
Cookie Name: AMCV_###@AdobeOrg
Unique visitor IDs used by Experience Cloud Solutions.
This cookie is used to identify a unique visitor
This cookie is stored at the domain of the website (not the domain of the image request).
Varies, most customers can expect this cookie to be around 200 bytes in length.
Please be aware that the cookies expiration can be changed in the implementation from the default expiration using:
SiteCatalyst determines unique visitor information using several technologies. The primary method of calculating unique visitors is by setting a persistent cookie on the visitor's browser to uniquely identify the visitor. Cookie technology helps to avoid common pitfalls, for example, IP Pooling, caching, or tracking visitors behind a firewall,when counting unique visitors. For more information on IP Pooling, If the visitor has disabled cookies on their browser, or if the visitor's browser does not support persistent cookies, Omniture uses a combination of the IP address and the user agent string to determine if a visitor is unique or not. SiteCatalyst reports a small percentage (usually 1-2%) of visitors who do not support cookies.
In your case Visitor is not deleting any cookie but Visitor profiles are getting deleted after 1 year of inactivity regardless of any visitor ID cookie expiration so the persistent cookie s_vi will take the precedence.That means your visitor will be a repeat visitor as persistent cookie is valid for 5 years.
Right, cookies are stores years unless the user don't delete them.
You could setup a segment with the definition that the visitor may not have been at your site in 6 months, 12 months or 2 years for example. Otherwise, if you want to just take the visitors coming that can't be recognised, then setup a segment where you take in the dimension 'Visit Number' and set that to 1, then you have all first-time visitors.