using awk to print line number with output lines

Question

context: grep for multiple strings in file on different lines (ie. whole file, not line based search)?

egrep -nH 'Dansk|Norsk|Svenska' file3

or using ripgrep

rg  'Dansk|Norsk|Svenska' file3

But there is an answer

awk '/Dansk/{a=1}/Norsk/{b=1}/Svenska/{c=1}END{ if (a && b && c) print $0 }'  file3

I don't understand this answer.

I understand awk '/Dansk/{a=1}/Norsk/{b=1}/Svenska/{c=1} .

I think now the question is using awk, while pattern matched, print line number with output lines.

update: I even understand this answer. update:
almost close:

awk '/Dansk/{a=1}/Norsk/{b=1}/Svenska/{c=1}{ if (a || b || c ) print $0; next}1'  file3

it return

Subtitle: 04, Language: da - Dansk, Content: Undefined, Stream id: 0x23,
        Subtitle: 05, Language: de - Deutsch, Content: Undefined, Stream id: 0x24,

the expected return is

   Subtitle: 04, Language: da - Dansk, Content: Undefined, Stream id: 0x23,

file3 content:

Disc Title: unknown
Title: 01, Length: 01:33:37.000 Chapters: 33, Cells: 31, Audio streams: 04, Subpictures: 20
        Subtitle: 01, Language: ar - Arabic, Content: Undefined, Stream id: 0x20, 
        Subtitle: 02, Language: bg - Bulgarian, Content: Undefined, Stream id: 0x21, 
        Subtitle: 03, Language: cs - Czech, Content: Undefined, Stream id: 0x22, 
        Subtitle: 04, Language: da - Dansk, Content: Undefined, Stream id: 0x23, 
        Subtitle: 05, Language: de - Deutsch, Content: Undefined, Stream id: 0x24, 
(...)

update. almost close.
awk '/Dansk/{a=1}/Norsk/{b=1}/Svenska/{c=1}{ if (a || b || c ) print NR, $0}' file3 But it returns

6         Subtitle: 04, Language: da - Dansk, Content: Undefined, Stream id: 0x23,
7         Subtitle: 05, Language: de - Deutsch, Content: Undefined, Stream id: 0x24,
8 (...)

Answer 1

awk is consist of pair of condition(s) & action(s) which condition part can be any arithmetic expression or pattern matching or result of one-or-more expression which all of these result can be true or false statement;

and here, the given sample is used a pair of pattern matching and actions which is:

awk '/if-this-regex-exist-in-current-line/ { "yes, so do something" }' infile

all of 3 doing the same pattern matching and set a temporary variable flag if those regex found in a line.

then at the END{} block, which this block only runs once after all records/line of the input infile file processed, which it simply checking if those all flags were set (means all 3 patterns were found either those were found in a single line or in while input file), then print the last line (you might wanted to print FILENAME instead or something else) of the input file.

---

Answer to the second added question about:

awk '/Dansk/{a=1}/Norsk/{b=1}/Svenska/{c=1}{ if (a || b || c ) print $0; next}1' file3

this prints the lines where that line was contain at least one of Dansk or Norsk or Svenska in it, otherwise print the line which all what this commnad do can be written awk 1 infile or cat infile.

In x ||y ||z || ... means "if x OR y OR z OR ... at least one were True" then do something while in x && y && z &&... means "if x AND y AND z AND ... all of them were True" then do something; "True" means a result of an expression(s) or a condition(s) evaluated true result.