Hi Paulo Manuel,
For reports, you need to add the widget monetary. Yes here also you need to add the related currency field.

Please add the t-options in the field of the report just like :

t-options='{"widget": "monetary", "display_currency": o.currency_id}'

For invoice you can add it in the price unit field of the report :

<span class="text-nowrap" t-field="line.price_unit" t-options='{"widget": "monetary", "display_currency": o.currency_id}'/>

Please vote the answer if it helps you.
Thanks