Apple support's online article:
Share photos and videos on iPhone - Apple Support
says:
"Share multiple photos or videos:
When viewing a screen with multiple thumbnails,
tap Select,
then tap the thumbnail of the photos and videos you want to share.
Tap
, then choose a share option such as Mail, Messages, or AirDrop".
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Seems another good way to send multiple large files smoothly and quickly is to create a .zip file
ie: This compresses multiple files into one smaller file to send.
However, you may need to save photos to the Files app and create a .zip file from there.
Apple support's online article (link below) explains how to to create a .zip file, and how to open a zip file if you receive one.
How to open ZIP files on your iPhone, iPad, or iPod touch - Apple Support