You can find a particular account’s posts using a particular hashtag by using the following web address:
(Mastodon account’s web address)/tagged/(hashtag without #)
For example if you wanted to browse posts with the hashtag #pixelfed made by the FediTips account, the address would be:
https://mstdn.social/@feditips/tagged/pixelfed
If you’re not sure what an account’s web address is, go to the account’s original page and the address will be shown at the top of your web browser.