Will there be conflicts in SEO if you just put links to city subdomains in the popup window?

Question

Search Engine Optimization

Hello everyone, there are a lot of regional subdomains, we want to make a popup window with links to other cities.
Will there be problems with SEO if people go from the main site to subdomains, in which 70% of the content is the same, except for addresses, product prices, and some city-specific things.

Answer 1

Where you put the links, in principle, does not matter much.
It's strange that they chose to implement it through a popup window, they usually make a drop-down menu in the site header, and the popup window pops up only with the text about the proposal to change the city .. or I just misunderstood you .. But that's not the point.
For the second part of the question: 70% are
the same

• Contact information on the site (in the header and footer of the site on each subdomain, designed with shema.org micro-markup);
  
• comply with the conditions of content substitution, namely: in meta tags, headings H1, ALT, text, but here you need to be careful, because frequent use of the toponym can be regarded as spam;
  
• We do the substitution on all pages and sections / subsections (do not touch meta tags and H1 of product cards).
  
• If necessary, you need to create several variables with cities in different cases.
  

It should also be taken into account that a frequent occurrence is the poor ranking of duplicate content on subdomains in Google (adding unique content corrects the situation).

What is often forgotten:

1. go through the "buyer's path" on each subdomain and check whether all the necessary changes have been made in the "Basket" and when placing the Order (often they forget to change the city)
   
2. check the correct formation of links in the sitemap.xml of each subdomain
   
3. implement a transition from the corresponding internal page of one domain to the corresponding page of another.
   
4. make settings in Yandex. webmaster and google console