Suppose I’m a lawyer executing a will, and I have video evidence that the will I hold in my hand is the latest authorized will of the deceased. If I hold the will up in my hand and say to the descendents of the deceased that "this legal document alone is relevant to the dispensing of the property of my client," the descendents might ask me how I know that, but I don’t have to answer their question by looking in the will itself. Of course, that is one option. Perhaps there is a notarized note and date in the will, or perhaps the will itself explains who and how one can recognize it as such. But I can alternatively provide, say, video evidence for my knowledge-claim, assuming I have it.
Suppose I’m a Christian preaching Scripture, and I have extra-biblical evidence that the Scripture I hold in my hand is the extant extent of divine revelation. If I hold Scripture up in my hand and say to Roman Catholics that "this document alone is the extant extent of divine revelation and, thus, our solely ultimate, authoritative rule of faith," Roman Catholics might ask me how I know that, but I don’t have to answer their question by looking in Scripture itself. Of course, that is one option. Perhaps there is a self-attesting claim in Scripture, or perhaps Scripture itself explains who and how one can recognize it as such. But I can alternatively provide extra-biblical evidence for my knowledge-claim, assuming I have it.
Moreover, this evidence needn't be infallible in order for it to serve a certain apologetic function. Videos can be fabricated and, say, historians - indeed, Christians - can differ, but we all must operate on assumptions in order to get anywhere in our lives. If we get to a point where we share assumptions with others, we have an easier time collectively following a train of reasoning to the same conclusion. So, for example, there can be value in historical theology, and not merely in the research of church historians (which can of course be useful), but also in the examination of the body of Christ itself.
Now, how we can identify theologically correct historians, believers, or human authorities prior to the identification of divine revelation - if such is possible - are appropriate questions that require an answer unless one is willing to admit we can know divine revelation apart from such external evidences, useful as they are. Elsewhere, I argue we can indeed identify Scripture apart from such evidences (link).
The main point of this post, however, is that Roman Catholics often do but should not conflate a metaphysical claim with an epistemological one. What something is and how we know what that something is are distinct questions. Sola scriptura is a metaphysical statement of what Scripture is - the extant extent of divine revelation and, thus, our solely ultimate, authoritative rule of faith - not an epistemological statement of how we know what Scripture is. Certainly, what Scripture metaphysically is may and does inform how we can know what it is, but I equally certainly don’t have to answer the question of how I know what Scripture is by looking through Scripture for a table of contents.