How do I calculate field_1 + field_2 in Twig? I have two fields. {{ content.field_1 }} is 2; {{ content.field_2 }} is 5.

How do I multiply them to obtain 10, in a template file?
In {{ content }} are render arrays which produce the output "2" and "5", but you can't do arithmetics with these render arrays.

You can try to do this with the database values from the node (if it is a node template):

{{ node.field_1.value * node.field_2.value }}