I can’t point you to YouTube videos…
This should be the last step.
Outside of Audacity there is ReplayGain which works in some player software, or Apple has Sound Check. There is also WaveGain and MP3Gain which make “permanent” changes to the audio files so they work with any software.
Audacity has Loudness Normalization but it doesn’t automatically check for clipping (distortion) if you apply too much amplification, so that’s something you have to watch for yourself.
ReplayGain and related do check for clipping and if they can’t boost enough to hit the target without clipping, they will only boost as much as possible without clipping.
Because most commercial digital music (including quiet sounding songs) is already normalized/maximized, it often can’t be boosted without clipping. For that reason, the volume-matching algorithms use a loudness target that ends-up lowering the volume of many/most tracks. If you have enough analog gain that’s not a problem but it bothers some people.
The process of cutting and playing vinyl changes the wave shape, making some peaks higher and some lower. If you normalize (maximize) for 0dB peaks, the average/RMS level will be lower than an original digital version and it will sound quieter than an original CD version. (To some extent the same thing happens with MP3.)
Plus, older vinyl was made before “digital weapons” were available for Loudness War so they are simply aren’t as loud as most modern digital recordings (even after peak-normalizing).
Do you have good speakers or good headphones?
The best thing is to use a known-good reference recording and then use the Graphic EQ by-ear. Without a reference it’s easy to get carried-away and over-boost the bass or highs and you might not notice until a later time that you’ve over-done it.
There are “matching EQ” effects, but you’ll have to search because that’s not built into Audacity. And you’d still need to check the results and maybe make some tweaks.
“Under driven” isn’t really a thing with digital, although it might not be loud enough for you.
If the original file is clipped/overloaded there isn’t much you can do. There is a Clip Fix effect, but it’s impossible to know the original wave height or shape so it’s imperfect. When I tried it a couple of times the wave looked better but didn’t sound better.
Audacity can Show Clipping. Note that it’s showing potential clipping. It’s just looking at the peak levels, it’s not looking at the wave shape.
Audacity can go over 0dB internally, so for example, if you boost the bass and it “shows red”, it’s not really clipped yet and if you lower the volume before exporting, you’ll be OK.
The Amplify or Normalize effects will bring the volume up or down for 0dB (or near 0dB peaks) so running either of those (with the default settings) will automatically bring the volume to a safe level. Usually, it’s a good idea to run one of those as the last step, but of course that will mess-up any previous loudness adjustments.
45’s often had lots of distortion and you can’t do much about that.
You also can’t un-do any “damage” done by low-bitrate MP3 compression.
P.S.
Personally, I only digitize vinyl when I cant’ find a digital copy. You rarely get “digital quality”.
Audacity as 3 tools for fixing-up vinyl clicks & pops. Click Removal (automatic), Repair (manual), or as a last resort you can zoom-in and re-draw the waveform.
There are also specialized applications - I have Wave Corrector which is FREE and automatic and Wave Repair ($30 USD) which is manual. Wave Repair does an audibly perfect job on most (but not all) clicks & pops, and since it’s manual it only “touches” the audio where you identify a defect. But it’s VERY time consuming.