1. A basic shelter, which consists of a workbench, a fire, a bed, walls and roof will allow you to sleep at night and update your spawn point. I often put up these mini bases around the map as I explore on foot in early game. Note that fires need to be sheltered from the rain and the smoke needs to vent. Later on the raids get tougher and you will have more crafting benches and upgrades which will require better defenses and more room. By then you will have portals.

2. The world is not infinite, although you can have (nearly) infinite worlds. Characters and worlds are saved separately so your character can log in to any world you have active. It is extremely large and it is unlikely you will explore all of it in one playthrough unless you really really want to. Each world is procedurally generated so you will have to explore to find things. (There's also a website that will show you a map if you enter your seed code if you really want to know where something is.) The player has a map with a fog of war and you can mark it when you find resources or put down a base or portal. (One thing that is not like Minecraft is that you can not dig underground, unless you burrow under a rock or other existing structure. The earth will go down to a certain depth but it will not cover your head.)

3. If you die you leave a tombstone with your stuff in it and a mark appears on the map. You will wake up at your last spawn point, which is you most recent bed. If there is no bed you go back to the start location. Keep extra food and supplies handy at your base. You also lose 5% of all your skills, even if that means losing entire levels, however, skills are not as important as equipment and food, so you might not even notice the difference.

4. Should be fine.

5. Portals are your friend. Also buildings can be deconstructed to get back all materials, even if they get damaged in a raid. You'll mostly be limited by the carry weight, and the inability to bring metal through the portal. Some things, like the forge, use ingots as an ingredient, so you'll have to move those by boat or foot, or mine more metal. It's not hard to put up a quick smelter and forge next to a mining vein in the new location if it comes to that. You can also have more than one base set up for mining, farming, etc and portal between them. There isn't much need to move the whole thing.

6. Mods work fine, but I'd recommend looking for ones that have been updated this month because we recently had a big patch. Just play it vanilla first to see if there's anything that really bugs you. Nexus mods with Vortex mod manager works great. Valheim Plus is well supported and has lots of stuff in it. If you don't like those death penalties or that no metal in portals thing, it can take care of that for you. (Just had to put this in there because Paka went there first.)