Some websites autoplay embedded video content upon accessing the page. Similarly, some sites (such as YouTube) insert commercials on popular content - as this is how they monetise their site.
Some newer browsers, such as Firefox desktop, are now inhibiting autoplay content such that it won’t play unless explicity requested:
https://blog.mozilla.org/firefox/block-autoplay/?utm_medium=email&utm_source=email&utm_campaign=2019fxnews-en&utm_content=04102019
Safari for iOS remains limited in its options to control content content play/rendering - however, some content blocker Apps can augment Safari’s native capabilties; 1Blocker X is one example of a highly configurable content blocker, however, it won’t inhibit a websites intended autoplay content.
Other browsers for iOS are available. With research, you may find one that inhibits autoplay. Firefox for iOS might even catch-up with its desktop cousin.
Hope you find this helpful.