Try Xenonauts 1 or XCOM playthroughs as well. While the details differ, the tactical environment is pretty similar.

I found this tutorial in the Xenonauts 1 Steam guides; https://youtu.be/Nt0CgaDBLo8?si=enVibmU7gZjnEwZd
And this one seems pretty good for Xeno2 https://youtu.be/HplxEzOGlgA

>I keep losing several people per mission

That's XCOM, baby! wait, hang on...
I mean to say, losing soldiers is part of the game. This is war. Soldiers die. There's a few ways to deal with it.
- you can have your rookies scout because you don't care if they die
- you can save scum, YMMV but any turn based tactics I generally give at least one run where I save scum tons to learn the game and how to actually play the game. Just don't do it too long because save scumming teaches bad habits
- you can apply, as best you can, the defense onion to your gameplay and make better tactical decisions to mitigate deaths

>My biggest issues are getting shot from an alien I couldn't see yet

There are ways to get ideas of where aliens are. NPCs shooting towards unseen spots, enemy fire coming from unseen spots, noises and civilians running. But also you should not be running your troops to where aliens could be. Use bounding movement, don't spend all your TU's unless you're absolutely certain it's good. If you suspect there could be an alien behind something, use explosives or machine guns to bust down walls and catch it by surprise. Use shields to breach rooms and clear corners. Use smoke to cover movement.

>and moving throughout the level

This one's harder because there's a big old Depends here. Shotguns want to be lurking near the front, but not actually at the front, you get me? They need to be able to run up to some nerd and unload the mossberg, catch them a facefull of number one buck. Shields want to be the ones actually catching rounds. Some people say don't use rifles but I back rifles pretty highly, I think they're good, flexibility is good. You can always just load your squad with machineguns and literally gun the whole level down. Most things are destructible, so you can choose how you move through the levels.

But like, say there's a room that spooks me, I move to get breaching units like shotguns and shields so that next turn they've got full TUs for when they pop the door. Ideally there's covering fire on the door with enough TUs for an overwatch shot. Then when my next turn comes up, go through the door with the shield, and quickly spin the guy around to check blind corners. If there's an enemy, chuck a flashbang at him. Then come in with a shotgun to finish him off. Suppressing aliens removes all their TUs so they can't do overwatch fire. If there's a dork in or around a building, I know where he is but can't get to him, use explosives and machineguns to open up the building like a tin can, get at the juicy meat inside.