This post is not finished. At this time, I am using sfdx a lot, and instead of searching for good pieces of information everywhere on the web, I am just trying to get everything I need in the same place. So, I will begin with a couple of commands, and I will add some others, from time to time.

Create a project and authorize an org

sfdx force:project:create -n MyProject
sfdx auth:web:login --setalias my-hub-org --instanceurl

Retrieve components from a package

sfdx force:source:retrieve -p path/to/source
sfdx force:source:retrieve -m ApexClass     //Retrieve all apex classes
sfdx force:source:retrieve -m ApexClass:MyApexClass     //Retrieve a specific apex class
sfdx force:source:retrieve -x path/to/package.xml       //Retrieve components from package

Deploy components from a package

sfdx force:source:deploy --manifest path/to/manifest/manifest.xml -l RunSpecifiedTests -r TestClass1,TestClass2 -w 33 --verbose --loglevel fatal -u username

Run apex tests

sfdx force:apex:test:run --classnames "TestA,TestB" --resultformat tap --codecoverage
sfdx force:apex:test:run --tests "ns.TestA.excitingMethod,ns.TestA.boringMethod,ns.TestB"