Update main and second table with a before update trigger for message logging I currently have the following testing schema:
CREATE TABLE userMessage (
    message_id BIGSERIAL PRIMARY KEY,
    creator_id BIGINT NOT NULL REFERENCES &quot;User&quot; (id),
    created_at BIGINT NOT NULL,
    modified_at BIGINT NOT NULL,
    content TEXT NOT NULL
);

CREATE TABLE userMessageLog (
    message_id BIGINT PRIMARY KEY,
    modified_by BIGINT NOT NULL REFERENCES &quot;User&quot;(id),
    modified_at BIGINT NOT NULL,
    content TEXT NOT NULL,

    foreign key (message_id) references userMessage (message_id)
);

Where who created the message, when and it's actual content. Then another table where I keep track of who modified (can be another user) the message, when and the previous content.
I also created the following trigger for when the message is first created:
CREATE OR REPLACE FUNCTION new_message() RETURNS trigger AS $$
    BEGIN
        INSERT INTO userMessageLog(message_id, modified_by, modified_at, content) VALUES (NEW.message_id, NEW.creator_id, NEW.created_at, NEW.content);
        RETURN NEW;
    END
    $$
    LANGUAGE plpgsql;

CREATE TRIGGER new_message AFTER INSERT ON userMessage FOR EACH ROW EXECUTE PROCEDURE new_message();

Which works as expected. As soon as something is inserted on userMessage a new log entry will appear on userMessageLog.
My question is, when the user or another user edits that message, how can I create a trigger and modify the content, modified_at columns from userMessage table and add a new entry to the userMessageLog? Is there a more better approach to this schema/trigger design?
My only approach resulted in a failed attempt of creating a trigger on BEFORE UPDATE for userMessage which would make the entry on userMessageLogand then UPDATE the userMessage table, but that would result in recursive calls because of the trigger and I also have no way of letting it know who modified it for modified_by and when it was modified for modified_at column.
First of all, if you want to add a new entry log for each insert/update operation, you should change userMessageLog's primary key. Otherwise you'll receive an error message due to duplicity.
Then, you can deal with trigger recursion by using: pg_trigger_depth(). You can get some examples in this questions:

Prevent recursive trigger in PostgreSQL.
Or at this fiddle I've set up using your table definitions.

Having said that, you can find another questions with some discussions about the use of pg_trigger_depth(), with points for or against of using it. Have a look at the answers of Evan Carrol and Erwin Brandstetter in this question:

Is pg_trigger_depth() bad to use for preventing trigger cascading (recursion)?

Personally, I'll use a function instead of a trigger.