First up, thanks for updating the question, @WorkingRicardo, that is really great information and helps a lot. 🙂 👍
I've made something that slightly resembles your flow, here:

If you want you can make yours very similar to that, by adding an action after your 'Enviar una solicitud HTTP a SharePoint' action, going to the 'My clipboard' tab, and then paste this in:
{"id":"b5238f28-d447-40e4-a724-514bd13caa33","brandColor":"#484F58","connectionReferences":{"shared_sharepointonline_1":{"connection":{"id":"/providers/Microsoft.PowerApps/apis/shared_sharepointonline/connections/shared-sharepointonl-70a14524-6d74-4196-b1fc-31d350117f82"}},"shared_sharepointonline":{"connection":{"id":"/providers/Microsoft.PowerApps/apis/shared_sharepointonline/connections/d77dac51a9a54000bd7e766fd7769f04"}}},"connectorDisplayName":"Control","icon":"data:image/svg+xml;base64,PHN2ZyB3aWR0aD0iMzIiIGhlaWdodD0iMzIiIHZlcnNpb249IjEuMSIgdmlld0JveD0iLTQgLTQgNjAgNjAiIHhtbG5zPSJodHRwOi8vd3d3LnczLm9yZy8yMDAwL3N2ZyI+DQogPHBhdGggZD0ibS00LTRoNjB2NjBoLTYweiIgZmlsbD0iIzQ4NEY1OCIvPg0KIDxwYXRoIGQ9Ik00MSAxOC41di03LjVoLTMwdjcuNWg1LjY0djEzLjgzbC0zLjI4NS0zLjI4NS0xLjA2NSAxLjA2NSA0LjAzNSA0LjA1Ljg3Ljg0aC02LjE5NXY2aDEzLjV2LTZoLTYuOWwuODU1LS44NTUgNC4wMzUtNC4wNS0xLjA2NS0xLjA2NS0zLjI4NSAzLjI4NXYtMTMuODE1aDE1djEzLjgzbC0zLjI4NS0zLjI4NS0xLjA2NSAxLjA2NSA0LjAzNSA0LjA1Ljg3Ljg0aC02LjE5NXY2aDEzLjV2LTZoLTYuOWwuODU1LS44NTUgNC4wMzUtNC4wNS0xLjA2NS0xLjA2NS0zLjI4NSAzLjI4NXYtMTMuODE1em0tMjguNS02aDI3djQuNWgtMjd6IiBmaWxsPSIjZmZmIi8+DQo8L3N2Zz4NCg==","isTrigger":false,"operationName":"Condition_MoreThanOneResult","operationDefinition":{"type":"If","expression":{"greater":["@length(body('Enviar_una_solicitud_HTTP_a_SharePoint')?['d/results'])",0]},"actions":{"SelectResultsToHtml":{"type":"Select","inputs":{"from":"@body('Enviar_una_solicitud_HTTP_a_SharePoint')?['d/results']","select":"@concat(\r\n '<p><strong>', \r\n item()?['createdDate'], \r\n ' - <em><a href=\"', \r\n item()?['author/email'], \r\n '\">', \r\n item()?['author/name'], \r\n '</a></em></strong><br/>', \r\n item()?['text'], \r\n '</p>'\r\n)"},"runAfter":{}},"JoinHtmlForCommentarios":{"type":"Join","inputs":{"from":"@body('SelectResultsToHtml')","joinWith":"\n"},"runAfter":{"SelectResultsToHtml":["Succeeded"]}}},"runAfter":{"Enviar_una_solicitud_HTTP_a_SharePoint":["Succeeded"]},"metadata":{"operationMetadataId":"a3f9c522-ebe8-4278-a6e0-58a55202d9bd"}}}
You can see the two Select actions there and you can also see that I am using a more accurate way to test if there are any comments. The expression in the left side is calculating the amount of comments for the list item:
length(body('Enviar_una_solicitud_HTTP_a_SharePoint')?['d/results'])
Then the condition and right side check that the number calculated is greater than 0.
All you would need to do after this is to place your 'Actualizar elemento' ('Update item') action and in the 'commentarios' value add the output from the 'JoinHtmlForCommentarios' action that I made.
It might look something like this, then:

The 'SelectResultsToHtml' action is where I did a bit of clever stuff using the following expression with a concat() function connecting together pieces of text and values from the comments request results into one HTML paragraph.
concat(
'<p><strong>',
item()?['createdDate'],
' - <em><a href="',
item()?['author/email'],
'">',
item()?['author/name'],
'</a></em></strong><br/>',
item()?['text'],
'</p>'
)
Because this sits in the 'Map' section of the Select action, it can address each item() programmatically. To select an individual key's value with the item() funtion you would use something like item()['keyName']. This would then place the value of that key name into the produced Select.
Even though the above looks complex, it is actually making a simple array, with only text entries. Each entry will correstpond to the comment that was entered, and makes its own '<p>' information for the HTML. 🙂