Research first. Then buy.
Harper's corollary: Buy the wire LAST.
Fire your purchasing agent. In fact, don't have your new guy purchase anything until the research is complete.
I bought this Siemens Meter-Load Center Combination, 12 Space, 6 Circuit but there are not any lugs
Yeah. Back it goes.
Your application is common (at least in North America): sprawling compound, service on a pole, and the desire to web power out to several outbuildings while also sending the whole kaboodle to the house. We have the perfect panel for that, it's called a ranch panel. Meter + main breaker + 8 breaker spaces + the thru-lugs to send 200A to the house. Here's one with meter pan and one without. These things are absolutely perfect for your application - it's what they're designed for.
However... whatever you use for a meter pan (integrated into the ranch panel, or separate) must be on the list of models approved by the power company. They are your first stop while shopping. If it's not on their list, the power company truck will drive away.
and then install a 100amp double pole breaker (providing total 200amp)
That is not how that works. A 100A double-pole breaker is 100A. Just, it's 100A at 240V instead of at 120V. It's weird, here's a primer.
to send the SE cable out
Say whaaaaa?? No, you can't use SE cable underground or hung from a pole line, so I do not see how would get the power to your buildings. You need something else like XHHW in conduit underground (18" cover for PVC, 6" for RMC), or MH feeder direct burial at 24" of cover underground, or quadplex on a pole line. And you will need 4-wire cable.
Are you speccing these things because you are studying exactly what was done with the last installation? Don't. It's a tire fire compared to modern code.
Rule of Six is dead for services
I guess just operates on the 'less than 6 switches' rule for NEC and has no main disconnect.
OperateD past tense. This is probably done in the 60s or 70s when there was a different sensibility toward Rule of Six. Back in the day, 100A+ breakers were extremely cost-prohibitive, so up to six 15-60A breakers could serve as the main breaker. Under the old rules they could even add up to more than the service size, if the dwelling Load Calculation validated. Trouble was, someone would have to re-do the Load Calculation every time they added anything - no one did, and panels burned. So that rule is gone.
The only way multiple breakers are allowed for a service is in 300A and larger services, because breakers larger than 200A are cost prohibitive. So a 400A service is dual 200s. They are not paralleled, they must feed 2 separate panels.
The AHJ's view will be that since ranch panels are readily available at sane cost, they won't authorize one of these.
Rule of Six continues to be allowed for subpanels, and is allowed to be oversubscribed because there is a main breaker upstream.
Don't "optimize for cheap" on panels
I couldn't imagine why you bought a $500 panel over a $350 ranch panel, until manassehkatz explained that the mail-order price is highly distorted and it normally goes for $125-ish. Ahhhh, got it. It was cheaper.
The future subpanel will have maybe 10 different breakers or so: AC, light zones, dryer, electric stove, various outlets, hot water heater, maybe a car charging port in future, etc.
Right, and you want the cheapest panel that will do the job. That thing there? Don't do that. That leads to the #1 panel problem we get around here: "Help! My panel is full because I did not foresee future needs!"
And by the way, you are wildly underestimating the breaker spaces needed for a dwelling. You are assuming that if you can't imagine it, it doesn't exist. The kitchen alone can account for 10 spaces. We regularly see dwellings fill a 40-space panel.
To make matters worse, you can't really use tandems in California, so that "20 space 40 circuit" number is useless.
And the way panel pricing breaks out, spaces are cheap up to 42 spaces. So we recommend a 40- or 42-space panel for literally every dwelling. You'll begrudge us the extra $100 now... but you WILL thank us later.
By "later" I mean early into the build when the AHJ explains how many circuits you actually need. Also, are you flipping this place or do you actually plan to live there? Because an even bigger blunder we see is the guy who is penny-hoarding while building, but creates an unlivable house: The girls can't run their hair dryers at the same time because somebody put all the bathrooms on the same circuit. Can't cook a big-family breakfast because somebody fit only 2 kitchen receptacle circuits. Electricity is our servant, it's there to serve us, not make us dance around its limitations. Romex is cheap. NEC is the slumlord bare minimum, below which the home is unfit for human habitation. It's not a design guide.
Don't worry, there are lots of other places for "optimizing for cheap" that we'll be happy to help with. Aluminum wire is top of that list, and come on back when you're ready to do that EV circuit and we'll save you a bunch there too.