There are two approaches to sending an email with attachments. The first is to share one or more items using an app that created or manages them and the second is to compose an email in the Mail app and attach one or more items using the Mail toolbar.
The process differs between apps but you should be able to find the share button in other suitable apps. In some apps a more button may reveal a share button. I'll give the details for Pages, Files and Photos here and similar processes can be used in other apps which produce files that can be shgared.
In Pages, open the document you want to send. A Share button is available in the top row of buttons. Double tap the share button. Pages is a special case since you may wish to convert the document before attaching it into a different format from Pages format, which isn't comonly useable on non-Apple kit. If you want to send the file in Pages format to another Apple user then select and double tap Mail in the second row of buttons in the share sheet that appears. You will then be composing a message with your Pages file as an attachment. If you want to send the Pages document in a different format, select and double tap Export And Send in the share sheet's third row of buttons. Swipe through the list of formats available and double tap your choice; PDF is best if the recipient doesn't need to edit the document; use Word if the recipient needs to edit the document. Once you double tap the format a new share sheet appears for the converted document and now you can use the Mail option in the second row of buttons to begin composing a message with the converted document as an attachment.
In Files, open the file you want to attach. There will be a Share button at bottom right of the screen. Double tap the Share button and then double tap Mail in the second row of buttons in the share sheet that appears. You are now composing a Mail message with the file as an attachment.
The Photos app allows you easily to share either a single or multiple photos or videos. To send a single photo or video as an attachment open the item and there wil be a Share button at bottom left of the screen. Double tap the Share button and then double tap Mail in the second row of buttons in the share sheet that appears. You are now composing a Mail message with the photo or video as an attachment.
If you want to send multiple items from the Photos app, use the Select button at top right of the screen and then locate and double tap the items you want to send. Now locate and double tap the Share button at bottom left of the screen. Locate and double tap Mail in the second row of buttons in the share sheet that appears. You are now composing a Mail message with the selected items as attachments.
The second approach to sending attachments begins with the compose button of the Mail app. Compose an email in the usual way. While entering the message body you can add your attachments.
A toolbar is positioned above the keyboard when you are editing the message body. Slide a finger above the top right of the keyeboard and you should find the Insert Drawing button. Slide your finger to the left to locate the Insert Attachment Actions button and lift your finger. Double tap. This pops up a menu. Swipe right if necessary to locate Attach File or Photo Library as required. Photo Library lets you select multiple items from your recent photos but Attach File allows only one file to be selected. I've recommended sliding a finger to locate the Attachment Actions button because swiping left and right through the toolbar may not be available on your device.
Finally, send the message as usual.
The toolbar will also be available when you are replying to an email.
If a document has been created in apps which use their own formats, such as Pages, Numbers or Keynote, you may need to export the document into a different format for it to be easily accessible to all recipients. PDF is often the most appropriate choice unless the recipient needs to be able to edit the document.