Only all articles and categories can be shared.

But each site can have "own" menu items and display only site-specific categories and articles via site-specific menu items.

Please check first two case studies:
https://alterbrains.com/joomla-extensions/administration/multisites-manager/case-studies

... be shown the latest articles published in the slave sites and also some single articles (always from slave sites) and there will be a menu with shared items.
Clicking on a shared category present in the menu will load all the articles that have that category of all the slave sites and clicking on ...

... from one single table from main database
Also, I want to know if we define some custom fields for customers (like age, location, etc) these can be shared ?

I think most appropiate way is the case study "Subsites with shared components data and independent core" but there you forgot to mention what ...

Only all menus and menu items can be shared.

Also, contents on some of the subsite may increase considerably, should I expect performance issues on all the portal if using a single installation?

Sure, it's same as running multiple sites.

... mainsite.com: displays some specific content
content2.mainsite.com: displays some other specific content

users and authentications should be shared
user will have different permissions on different sites (ie can post on content site but not on mainsite)
content, menus and templates will be ...

... database tables.

Note that both sites should use same MySQL server and MySQL user of slave site should have access to database of master site.

Note that images won't be shared, but slave site can have a symlink to master site's VM images instead of usual folder, hence all VM images will be shared.

... www.neothermic.com/kb/article55.html

So theoretically it's possible to install multiple copies of phpBB and configure the constants.php to use shared tables for authentication, authorization, sessions, etc. Since the document is dated, I need to do some research to discover if other tables need ...

... com

The domain has no forum but menu has links to forums in subdomains. The five subdomains need their own forums because category heirarchy is extensive. Content is shared among domain and subdomains. Domain hosts other resources such as directories, glossaries, classified ads, help system, etc.

The main requirement is that phpBB on each domain should display only own forums?

Can the phpBB users be shared?

I think it's possible to have single phpBB installation but display only certain forums depending on current domain name.

More control over sites with shared menus and contents:


⋅ Feature: new Overrides settings 'Hide Menus' and 'Display Menus' allows to hide and/or exclusively display all menu items from selected menus.
⋅ Feature: new Overrides settings 'Disallow Content Categories' and 'Allow ...