https://www.tdgarden.com/the-garden/dining/
Club and lower suites is level 5, and you get access to the mini-concourse of level 6 (upper deck of suites) as well - level 4 is the top part of Loge. You get access to all of that - except for the Sports Deck (free appetizers and soda) and the Boardroom (all you can eat, I think?). The "Sports Museum" is basically the concourse of 5 and 6 - you can check all of that out, and see the old Garden seats, for example.
You'll get access to the cafeteria-style "Premium Club Bistro" (man, that name's seriously up-selling it IMO) on east side of level 5 where you can literally watch Dale and whoever-else do the intermission commentary. I think there's a raw bar on level 5 or 6 that you can get to, too - Boston Coach lounge maybe? The "Bistro" basically has very similar food to what you'd find elsewhere in the Garden - sandwiches, pizza, chicken fingers, sausages, burgers, gelato. Maybe with slight variants - they have fancy fries, for example, and different burger toppings. They also have some tables where you can sit and eat...if you're coming pregame, you can easily grab dinner and sit there. Getting an intermission meal AND eating at one of the few tables there during the 17 minutes? Probably not gonna happen, because I would guess the tables get filled during intermissions.
You can also use the in-seat service to order food and drink - again, mostly the typical fare you'd see in the Loge/Balcony, and probably "ordered" from the Bistro.
Legends is on 3, I think - never been there, myself. Totally separate access.
https://tdgardenapps.com/legends/menu.html
If the menu is up to date, Sam Adams is 50 cents cheaper there than in the rest of the Garden! Yeesh. You can probably find Legends passes on Ace or StubHub, but I don't see any for tomorrow. I've never been there, but it looks like the kind of place you'd take a client you're trying to impress. Paying $30 to Ace Ticket for a pass (I think that gets 2-4 people in? Not sure...) on top of the charge for, say, $28 salmon? Color me slightly skeptical.