add custom title to category and pages products

I want to add additional text to title of pages.

example:

Category name | Additional text
or
Product name | Additional text
or
Contact us | Additional text

How to do that?

Opencart version 3.0.2.0

If you mean add additional text to all category pages and all product pages - You can use the custom html module built into opencart and add it using design => layouts

To add a new field, you will require a module or to create one yourself.

I have a flexible module that will allow you to do that individual product or category or page specific. It is built to be include specific pages or exclude specific pages.
https://www.opencart.com/index.php?rout ... n_id=26852
It is quite expensive for your purposes, but undervalued for its capabilities.
I am sure if you search you will find something in the marketplace - maybe even free.

Usually a SEO extension can help you with customizing page title, names and descriptions. For example, my Smart SEO extension can do this for you. You can create your own pattern for titles, using tags and adding custom text, if necessary. Something like this:

{name} - {store} | Cool text

Which will translate into this:

Product Name - My Store | Cool text