Here. Watch. Good info. I am torn on whether to be amazed or horrified that you managed to get that far in the game without knowing about defences. LOL

https://youtu.be/_C9fKFg6bmw

https://youtu.be/vrIigNKnFTA

You can. There a several structures that prevent spawns around them. Campfires/bonfires/hearths, workbenches/forges/stonecutters/artisanTable, etc. Wiki list can be read here. Cannot verify if complete or not.[valheim.fandom.com]

Umm, you have no base defenses? Have to say that it is on you if you choose to just wander around in the plains and make a farm with no way to protect it or you. You can trench it and you can make an earthen wall or do both. And no that is not cheese or exploit, it is a base function of learning to play the game. With historical backup. May I introduce you to walled cities, all over the world that protect that which is valuable from those who would like to ruin it. Even Vikings of yore walled their towns, fields.
Put down workbenches so that their area of security overlaps.
Or... you can hope to be lucky, and we know how that has worked out. Important lesson.

How did you get all the way to plains without understanding how to suppress spawns in your base?

If you know the structures that block spawns, you can. In many cases they are things the player would naturally place around their base in ways that would block spawns already. For example, you need workbenches to build the entire base, so there's normally workbenches still covering the entire base as far as protecting it to. Workbenches spawn block radius is the same as their build circle so they are a real easy way to make sure you got an entire area covered (I do believe this applies to all the build/crafting stations with a build radius as well, so things like stone cutter as well).

Besides that you generally want fire all over the base to easily maintain rested and those also block spawns. Another is the lights basically, like torches. Players love putting these all over just to see better at night, and they also block spawns near them (and no, you dont have to keep them lit to have this effect). Then we have the ward which helps protect the players stuff (and inform you that its getting hit in the first place if you are on the other side of a larger base). Another that blocks spawns that players love to place so they protect the entire base area, bonus that wards can actually be buried underground without just exploding (like workbenches can be partially buried, but if buried too much they just break instead...wards do not) so you can bury one in the dead center of your cultivated area underneath your farm and forget it even exists but spawn-block that entire area as well.

Plains bases especially need good defenses, which can sometimes be challenging to figure out when you have mechanics like earth walls being the default defense, but encasing your windmills can potentially block their winds (but hey, if the roof can support it there's no rule against putting them higher than your actual walls either). I personally like to make a wider trench outside of my plains home with a higher earth wall on the interior and put my tamed lox in the pit basically (the higher earth wall is because lox are apparently surprisingly stackable for whatever reason). Anything that falls into the pit cant really hurt my base and dies to my lox, i just occassionally toss some cloudberries into the pit for healing/breeding the lox, and if a few get out because of the stacking thing, oh well, i have a couple lox patrolling my borders until they get nuked by a deathsquito backstab or something (the pit loxes are usually lower stars, if i get 2 star lox, i treat them a bit nicer).

Edit: worth noting that the troll raid is possibly the worst with this particular setup. A log troll can take out a surprising amount of lox before they eat it. Its still best to just kite the trolls away on that event