NAICS codes describe what your business does — for example, a marketing agency claims a marketing code. You search for the codes that match your capabilities and claim them as part of your registration; they are not a barrier to getting started.
How they work in practice:
Opportunities carrying the codes you claimed show up in your systems and notifications.
A single solicitation can carry multiple codes — for example printing and marketing on one project — so broader coverage surfaces more matches.
One company can hold many codes covering different capability areas; you do not need a separate entity per service line, and holding dozens of codes under a single company is normal.
You can claim additional codes later if you want to pursue new lines of work.
Ultimately, your capability statement has to back up whatever you bid on — codes get opportunities in front of you, but demonstrated capability is what wins them.