I rely heavily on an Apple Shortcut that I made which saves an offline copy of a webpage (just the text and images) and stashes it in Notes. Great for refreshing myself on a topic I read about months ago but hasn't become relevant until the current moment.
I have recently started using Zotero, which can create offline copies of saved webpages and papers. You can then search their contents.
Last week I bookmarked Yacy, which is a search engine and which seems to allow you to define your own web index. However, I haven't gotten around to seeing how it really works...