VEVOR 6x6 Post Base 4 Pcs, Inner Size 5.6"x5.6" Post Base Brackets, Heavy Duty Powder-Coated Post Anchor Matte Black Wood Post Brackets for Pavilion Deck Railing Support Deck Base Plate

It calls for a 10mm. But I used a 3/8. Worked fine.

Can’t answer that but want to advise that when a 4x4 was placed in the units three of my five units’ welds broke. Would not purchase again. Toja grid units are more expensive but I had no issues with them breaking apart.

Not sure. I fastened my post into my wood deck with galvanized bolts and washers when I built a 10 x 14 roof on the deck. My guess - use a concrete drill bit with the same width as the lantern shade fastener you see on the pin. When you tap each pin down into the hole it should fit snugly. Then begin to tighten the bolt until it feel snog. Don't overtighten the pin bolt.