Display first image from matrix field of a related entry to current tags I want to display on entry view page, related entries to current tags.
So far, I came to displaying basic entry's object, but I don't know How to get in matrix field and pull the first image.

Images are stored in matrixFieldHandle -> blockTypeHandle -> imageFieldHandle.

For outputting related entries to current tags I use:

{# Set ECM for related entries to current tags #}
{% set tag = entry.keywords %}
{% set relatedEntries = craft.entries.relatedTo(tag).id('not ' ~ entry.id).limit(8).find() %}


{% for entry in relatedEntries %}
    {{ entry.title }}
{% endfor %}
I think entry.matrixFieldHandle.type('blockTypeHandle') is what you're looking for. Putting it together, it'd be something like:

{% set tag = entry.keywords %}
{% set relatedEntries = craft.entries.relatedTo(tag).id('not ' ~ entry.id).limit(8) %}

{% for relatedEntry in relatedEntries %}

    {% set matrixBlock = relatedEntry.matrixFieldHandle.type('blockTypeHandle').first() %}

    {% if matrixBlock %}

        {% set asset = matrixBlock.imageFieldHandle.first() %}

        {% if asset %}

            &lt;h1&gt;{{ relatedEntry.title }}&lt;/h1&gt;
            {{ asset.img }}

        {% endif %}

    {% endif %}

{% endfor %}