it all depends on where you go because on a federal level using marijuana recreationally is still illegal so the laws are left up to the individual states. how does this work? there are federal laws that all citizens must follow in addition to state laws that citizens can vote on if they are a citizen of that state and laws might vary from state to state based on which laws they vote for. like what The Network i'm sure if familiar with there is no sales tax in Oregon but their direct neighbor to the north (Washington) does have one. Oregon makes up for this deficit in other ways and from what i hear is not so tax friendly when it comes to that time of year on a business or personal level. but both Oregon and Washington allow recreational legal marijuana.
if the number of states that allow recreational marijuana use get to an overwhelming number and elect the the people into congress who share their views then the federal laws could be changed to make marijuana legal to use. that is slowly happening and time will tell.
this link has a map of which states can use it recreationally, medically and which ones are still illegal. something more complicated which is by limited THC content is something else a lil different.
https://www.vox.com/identities/2018/8/20/17938366/medical-marijuana-legalization-states-map
even though it's legal in states and our capital the District of Columbia (Washington D.C., yes 50 states and one district here) it's not legal to sell in all places. weird right.
in addition to what TLB and The Network have posted: they scan your state I.D. card in some states when you buy i hear. though selection is higher the same product is not always on the shelves a month later.
i know someone in one state who use to get quarter ounces for a fairly reduced price from the black market. trying to get it from a legal shop at that same price does not happen. they also told me the black market is now almost non existent.
someone i know from another state told me they have higher quality for selection then they use to get off of the black market though at a higher price. one would think the trade off is worth it but to each their own.
the states who have legalized recreationally have done so mostly through legal means of income for the state. they tax it though i'm not sure if the standards have gone up (what The Network says is true though).
basically all the states are making this up as they go along (sales, quality, taxes, etc.) because it has never been done here before and some are doing better than others. w01g4ng has a great state and good break down of things if they could be troubled to get their butt to post here.
there are other practical problems that come up that are relevant to this site perhaps but not so popular to talk about or not generally accepted by most people who use. when a police officer pulls you over for drunk driving a breathalyzer test can be given to determine the amount of alcohol in your blood. this is an interesting thing because the blood carries the alcohol in your veins to your lungs to be expelled and a device was invented that could give accurate measurements on the spot based off of how much you breath out. measuring the content of THC in ones system so accurately and quickly has not yet been invented. so when a person is driving and gets pulled over for DUI/DUII (driving while under the influence/ of intoxicants) of using marijuana the have to use pre-existing field tests (much like for drunk driving or other drugs, i.e. walking a straight line, pupil dilation, etc.) to test your ability to drive and if you fail then they can arrest you and while booking you can take a sample of urine (pee test, UA) to determine your exact content of THC.
each state is making this up as they go along as well and coming up with general guidelines as to what you can and cannot do when it comes to smoking and driving. best not to smoke right before you drive is obviously one of them. something like waiting an hour if you have been smoking a joint i hear.
one law in some states is that it's like alcohol consumption. you can use recreationally in the privacy of your own home but not smoke in public. this has led to some places being opened to general smoking but not much at this time.
even though it is legal to use marijuana recreationally in the state you live in your employer who is not under mandate by the state or federal governments might not hire you or allow you to continue to work for their company if they have a no drug policy.
this post is in addition to what was asked and already talked about as well as the way things generally work for posterity reasons but also to show how things are working themselves out.