Posts are entries listed in reverse chronological order on the blog home page or on the posts page (if you have set one in Settings → Reading). Posts do use tags and categories, and can be navigated through the tag/category navigation feature. If you have created any sticky posts, those will appear before the other posts. Posts can be found in the Archives, Categories, Recent Posts, and other widgets.
Pages are static and are not listed by date. Pages do not use tags or categories. An About page is the classic example.
Read more at WordPress Post vs. Page
A portfolio item is nothing else but a post, but with particular features. As posts, it may contain categories and tags. Additionally, the portfolio feature allows to present works in a standardized layout, with the additional function of an overview page (=”Portfolio”), which is mostly a grid, or a carrousel showing a preview ( “Featured image) of each portfolio entry. This “grid page” is the typical property of a portfolio site.
Note that some themes require “portfolio” items (like the theme “Espied”), while other ones require “project” items (like “Divi”). Check out the theme you want, and present your items in the required form.
Best is, create a series of dummy images, create a portfolio (and/or) a project page for each item, and then check out the themes. You can also download a set of 12 dummy pictures here (12 featured images, 12 regular images).
Also check out if a theme shows the featured image in the item page again. Some do, some not.
There are a few themes with portfolio feature installed in your CMS. More themes can be found at:
If you like Divi, there’s a tutorial at Youtube how to handle Divi portfolios.
What to use when
While pages are more for static, standalone contents, posts are more useful when you have a larger amount of smaller information entities to share. Portfolio/Project items are particular posts for presentning a series of work, pictures, etc.
The tag/category navigation allows the user to navigate contextually, a method that is considered as more intuitive than navigation by menus, at least for the more experienced web users.