For your example, 「ポーション」や「毒消し」は、たくさん持っていてそんはないぞ. The grammar points here are や, which like と, for listing stuff (non-exhaustively). Then I searched “tofugu や” and got Particle や For Listing Multiple Items . We have a couple of はs, you probably know how they work. You can see 持っていて is 持つ in its ている form, which is common and you can also search for : Japanese Verb Continuous Form ている . Another easy one is ぞ at the end. Anything weird at the end is either a sentence-ending particle (ぞ is a common one) or some kind of verb conjugation / auxiliary that you don’t know yet (see all the discussion of the weird freaking ん at the end of the sentence).
Then it gets a little more advanced, because 持っている is itself in て-form. You can tell because of the… て. So, search up て-form and you’ll find that it can be stuck on the end of a full sentence/clause, and acts basically as a comma, or an “and”, or a “then”, connecting two sentences, and it can also be a command (or a quote, but you can usually tell when something is being “quoted”). Honestly, I’m not sure which it is, because I think the next part is 損はない, which I would guess means something like “there is no harm”.
This is all complicated by the fact that a lot of the time, JP will just drop important pieces of grammar! Sometimes you don’t know that, actually, there’s supposed to be a に in there, or a comma, or a が, but they just left it out, and the meaning becomes super ambiguous. A more experienced person would tell you, probably, that you end up seeing the “weird” grammar patterns often enough that they aren’t weird anymore. That isn’t very encouraging for a newcomer, but it does get easier the more you read.
Finally, put it together the best you can. You don’t have to be perfect, and you don’t have to get it exactly. Ask questions. Is そんはない a common idiom? I don’t know. Is this guy telling me to carry a lot of potions, or is て connecting two clauses? If it’s the latter, what is the relationship between the two clauses that て is implying?
As you can see, I don’t 100% understand the grammar. But, I would venture that this means something like “There is no harm in carrying many potions and antidotes”. Then, some other guys will tell me I’m wrong, and we’ll all learn something =D