Before installing your WordPress Multisite, you should decide on the URL structure you want to give it. You have the choice of either a subdomain structure or a standard subdirectory structure through which the different pages of your multisite can be accessed. During the multisite installation, you will have to decide which of these two structures you want to use. It will not be possible to change it afterwards.
Example of a subdomain structure:https://website1.my-maindomain.com; https://website2.my-maindomain.com; https://website3.my-maindomain.com, etc.
Example of a subdirectory structure:`https://my-maindomain.com/website1`; `https://my-maindomain.com/website2`; ´https://my-maindomain.com/website3`, etc.
The URL structure is a key factor in search engine optimization (SEO). If you use a subdomain structure, each individual subdomain (e.g. `https://website1.my-maindomain.com´) is basically treated by search engines as a separate website. If you opt for a subdirectory structure, all websites in your WordPress Multisite network will be treated as part of a single (large) website. When using a subdomain structure, you thus have multiple (often smaller) websites which are independent of one another. With a subdirectory structure, on the other hand, search engines will recognize a single (often relatively large) website in your multisite network. Both types have their advantages and disadvantages with regards to SEO, which you should familiarize yourself with in advance. Other than that, keep in mind that search engines like Google also change their rules from time to time.